Disobedience is what comes to mind.
Perhaps "the desire of the body, the lust of the eyes, and pride of life" has something to do with it.
It was disobedience,and that is why there was that one tree there,because God gave them a choice,whether to obey Him,or do their own thing,and I do not believe that it would have entered their mind to disobey unless an outside source tempted them,so God allowed Satan to tempt Eve,and then she tempted Adam.When Satan tempted Eve by saying,did God really say that,she told him straight up they could not eat of that tree,so Satan tried a different approach by trying to get her to exalt herself.
For all that is in the world, the lust of the flesh, and the lust of the eyes, and the pride of life, is not of the Father, but is of the world(1 John 2:16).
And when the woman saw that the tree was good for food, and that it was pleasant to the eyes, and a tree to be desired to make one wise, she took of the fruit thereof, and did eat, and gave also unto her husband with her; and he did eat(Genesis 3:6).
Eve commited all 3 of those desires of the world,in that order they are described in 1 John 2:16.