I would argue that it is Mollie’s “foolish” nature that inspires her to leave the farm. From the start, she is portrayed as dainty, frivolous, and silly. She listens to not a word of Major’s speech. She becomes known as silly and materialistic when she enquires whether or not there will “still be sugar after the rebellion.”
Immediately after this, she showcases her frivolity by asking whether she will “still be allowed to wear ribbons in [her] mane.” In a nutshell, before the rebellion even begins, Mollie is showing herself to be an animal who will not fit into the new Animal Farm regime. During the Battle of the Cowshed, she proves her lack of mettle by going to hide at the first sound of gunfire. All these facts contribute to the image of Mollie as a character primarily interested in her creature comforts.
It becomes increasingly clear that Mollie enjoyed the comforts of having a human master. She wishes to hide this, however, and becomes very upset when she is caught having her nose stroked by one of “Mr. Pilkington’s men” from Foxwood. After confronting her, Clover finds a stash of ribbons and sugar lumps in her stall, and her secret is out.
It is soon after this that Mollie finally makes her departure from the farm, disappearing without a word. After a few weeks, the pigeons see her “on the other side of Willingdon,” in the company of a “fat red-faced man” who is obviously her new master. With her newly clipped coat, scarlet ribbon, and supply of sugar, it is evident that what caused her to leave Animal Farm was the fact that she prioritized creature comforts over the notion of revolution.