Matthew 24:29-31 is dealing with the rapture/resurrection not the second coming
The 7 seals that Jesus opens are for the tribulation to weed out the tares within His body by the end of the great harvest:
Matthew 13:30 Let both grow together until the harvest: and in the time of the harvest I will say to the reapers, Gather up first the tares, and bind them in bundles to burn them; but gather the wheat into my barn.
Matthew 13:36-43 Then he left the multitudes, and went into the house: and his disciples came unto him, saying, Explain unto us the parable of the tares of the field. (37) And he answered and said, He that soweth the good seed is the Son of man; (38) and the field is the world; and the good seed, these are the sons of the kingdom; and the tares are the sons of the evil one; (39) and the enemy that sowed them is the devil: and the harvest is the end of the world; and the reapers are angels. (40) As therefore the tares are gathered up and burned with fire; so shall it be in the end of the world. (41) The Son of man shall send forth his angels, and they shall gather out of his kingdom all things that cause stumbling, and them that do iniquity, (42) and shall cast them into the furnace of fire: there shall be the weeping and the gnashing of teeth. (43) Then shall the righteous shine forth as the sun in the kingdom of their Father. He that hath ears, let him hear.
This parable of the wheat and tares speaking of angels being sent forth to harvest at the end of the world is further proven in Revelation 14:
Revelation 14:14-20 And I saw, and behold, a white cloud; and on the cloud I saw one sitting like unto a son of man, having on his head a golden crown, and in his hand a sharp sickle. (15) And another angel came out from the temple, crying with a great voice to him that sat on the cloud, Send forth thy sickle, and reap: for the hour to reap is come; for the harvest of the earth is ripe. (16) And he that sat on the cloud cast his sickle upon the earth; and the earth was reaped. (17) And another angel came out from the temple which is in heaven, he also having a sharp sickle. (18) And another angel came out from the altar, he that hath power over fire; and he called with a great voice to him that had the sharp sickle, saying, Send forth thy sharp sickle, and gather the clusters of the vine of the earth; for her grapes are fully ripe. (19) And the angel cast his sickle into the earth, and gathered the vintage of the earth, and cast it into the winepress, the great winepress, of the wrath of God. (20) And the winepress was trodden without the city, and there came out blood from the winepress, even unto the bridles of the horses, as far as a thousand and six hundred furlongs.
The 6th seal at the end of the tribulation marks the beginning of the year of wrath AFTER the 7 years of tribulation:
Revelation 6:12-17 And I saw when he opened the sixth seal, and there was a great earthquake; and the sun became black as sackcloth of hair, and the whole moon became as blood; (13) and the stars of the heaven fell unto the earth, as a fig tree casteth her unripe figs when she is shaken of a great wind. (14) And the heaven was removed as a scroll when it is rolled up; and every mountain and island were moved out of their places. (15) And the kings of the earth, and the princes, and the chief captains, and the rich, and the strong, and every bondman and freeman, hid themselves in the caves and in the rocks of the mountains; (16) and they say to the mountains and to the rocks, Fall on us, and hide us from the face of him that sitteth on the throne, and from the wrath of the Lamb: (17) for the great day of their wrath is come; and who is able to stand?
Revelation 6:12-17 proves that it is not until the 6th seal is opened by Jesus that that the worldly are declaring "the great day of their wrath is come"!
The 6th seal causes a great earthquake, the sun becomes black, the moon becomes as blood, the stars fall to the earth, the heaven was removed as a scroll when it is rolled up, every mountain and island were moved out of their places, and they say to the mountains and to the rocks, Fall on us, and hide us from the face of him that sitteth on the throne, and from the wrath of the Lamb.
These same events are described as happening AFTER the tribulation in these three passages of the Olivet discourse when we examine precept upon precept:
Matthew 24:29-31 But immediately after the tribulation of those days the sun shall be darkened, and the moon shall not give her light, and the stars shall fall from heaven, and the powers of the heavens shall be shaken: (30) and then shall appear the sign of the Son of man in heaven: and then shall all the tribes of the earth mourn, and they shall see the Son of man coming on the clouds of heaven with power and great glory. (31) And he shall send forth his angels with a great sound of a trumpet, and they shall gather together his elect from the four winds, from one end of heaven to the other.
Mark 13:24-27 But in those days, after that tribulation, the sun shall be darkened, and the moon shall not give her light, (25) and the stars shall be falling from heaven, and the powers that are in the heavens shall be shaken. (26) And then shall they see the Son of man coming in clouds with great power and glory. (27) And then shall he send forth the angels, and shall gather together his elect from the four winds, from the uttermost part of the earth to the uttermost part of heaven.
Luke 21:25-28 And there shall be signs in sun and moon and stars; and upon the earth distress of nations, in perplexity for the roaring of the sea and the billows; (26) men fainting for fear, and for expectation of the things which are coming on the world: for the powers of the heavens shall be shaken. (27) And then shall they see the Son of man coming in a cloud with power and great glory. (28) But when these things begin to come to pass, look up, and lift up your heads; because your redemption draweth nigh.
The second coming is described in Revelation 19 and has no mention of all the events listed above but does directly deal with the judgment through wrath that the wicked feared by pleading that "the mountains and to the rocks, Fall on us, and hide us from the face of him that sitteth on the throne, and from the wrath of the Lamb: for the great day of their wrath is come; and who is able to stand?" in Revelation 6:16-17~ :
Revelation 19:11-21 And I saw the heaven opened; and behold, a white horse, and he that sat thereon called Faithful and True; and in righteousness he doth judge and make war. (12) And his eyes are a flame of fire, and upon his head are many diadems; and he hath a name written which no one knoweth but he himself. (13) And he is arrayed in a garment sprinkled with blood: and his name is called The Word of God. (14) And the armies which are in heaven followed him upon white horses, clothed in fine linen, white and pure. (15) And out of his mouth proceedeth a sharp sword, that with it he should smite the nations: and he shall rule them with a rod of iron: and he treadeth the winepress of the fierceness of the wrath of God, the Almighty. (16) And he hath on his garment and on his thigh a name written, KING OF KINGS, AND LORD OF LORDS. (17) And I saw an angel standing in the sun; and he cried with a loud voice, saying to all the birds that fly in mid heaven, Come and be gathered together unto the great supper of God; (18) that ye may eat the flesh of kings, and the flesh of captains, and the flesh of mighty men, and the flesh of horses and of them that sit thereon, and the flesh of all men, both free and bond, and small and great. (19) And I saw the beast, and the kings of the earth, and their armies, gathered together to make war against him that sat upon the horse, and against his army. (20) And the beast was taken, and with him the false prophet that wrought the signs in his sight, wherewith he deceived them that had received the mark of the beast and them that worshipped his image: they two were cast alive into the lake of fire that burneth with brimstone: (21) and the rest were killed with the sword of him that sat upon the horse, even the sword which came forth out of his mouth: and all the birds were filled with their flesh.