My main sim has a Festering Grudge toward her BFF but the game doesn't tell me what caused it.

I think saying no to BFF gives them creeped out sentiment, not festering grudge. Maybe they were playing football together without you noticing, because sims love to start this on their own if they have a ball. One bad hit and grudge is there. I had festering grudge between father and son just because of the stupid ball.