My speculation when he popped up was that the party had gone badly, and Rachel had been killed. They had told him about the Box ("So you would [tell someone else] if it were an emergency?") that they could go back and fix it, and he'd gotten stuck in one of the loops. Now that I think about it, the 'someone else' was in fact Rachel's Dad during that conversation. Since Aaron at that point was already looping through several times, I suspect he was almost-confessing to Abe, or thinking about telling the dad on another trip through to garner his support, and the scene in the car was him playing ignorant yet again.