Mat_8:28 And when he was come to the other side into the country of the Gergesenes, there met him two possessed with devils, coming out of the tombs, exceeding fierce, so that no man might pass by that way.
Mar 5:1 And they came over unto the other side of the sea, into the country of the Gadarenes.
Mar 5:2 And when he was come out of the ship, immediately there met him out of the tombs a man with an unclean spirit,
Mar 5:3 Who had his dwelling among the tombs; and no man could bind him, no, not with chains:
Mar 5:4 Because that he had been often bound with fetters and chains, and the chains had been plucked asunder by him, and the fetters broken in pieces: neither could any man tame him.
Mar 5:5 And always, night and day, he was in the mountains, and in the tombs, crying, and cutting himself with stones.
Mar 9:17 And one of the multitude answered and said, Master, I have brought unto thee my son, which hath a dumb spirit;
Mar 9:18 And wheresoever he taketh him, he teareth him: and he foameth, and gnasheth with his teeth, and pineth away: and I spake to thy disciples that they should cast him out; and they could not.
Mar 9:19 He answereth him, and saith, O faithless generation, how long shall I be with you? how long shall I suffer you? bring him unto me.
Mar 9:20 And they brought him unto him: and when he saw him, straightway the spirit tare him; and he fell on the ground, and wallowed foaming.
Mar 9:21 And he asked his father, How long is it ago since this came unto him? And he said, Of a child.
Mar 9:22 And ofttimes it hath cast him into the fire, and into the waters, to destroy him: but if thou canst do any thing, have compassion on us, and help us.
Mar 9:23 Jesus said unto him, If thou canst believe, all things are possible to him that believeth.
Mar 9:24 And straightway the father of the child cried out, and said with tears, Lord, I believe; help thou mine unbelief.
Mar 9:25 When Jesus saw that the people came running together, he rebuked the foul spirit, saying unto him, Thou dumb and deaf spirit, I charge thee, come out of him, and enter no more into him.
Mar 9:26 And the spirit cried, and rent him sore, and came out of him: and he was as one dead; insomuch that many said, He is dead.
Act 19:13 Then certain of the vagabond Jews, exorcists, took upon them to call over them which had evil spirits the name of the Lord Jesus, saying, We adjure you by Jesus whom Paul preacheth.
Act 19:14 And there were seven sons of one Sceva, a Jew, and chief of the priests, which did so.
Act 19:15 And the evil spirit answered and said, Jesus I know, and Paul I know; but who are ye?
Act 19:16 And the man in whom the evil spirit was leaped on them, and overcame them, and prevailed against them, so that they fled out of that house naked and wounded.
We can see how people that were possessed of a devil acted,crazy,mentally tortured,wild,fierce,and the spirit trying to cast them in to the fire,or the water to destroy them.
So no wonder the pigs ran in to the water and drowned,they were all a mess when they were possessed by an evil spirit,and went crazy,which they were probably trying to get rid of the way they felt,and the torment.,or else the evil spirits caused them to drown in the water,so they can escape after the pigs died.