Peeta could bake them a cake telling them to fuck off? Mmm angry cake filled with tracker jackers. 

It also stands to reason that as the Games get nearer, Haymitch and Katniss should be on edge so them both snapping could have something to do with that, not just Gale and Prim. But I can add something about Peeta and Gale trying to help but I bet Katniss would try to approach Haymitch on the train or he might approach her, considering all four are really stuck together now. Or Katniss would have some mental break down and Haymitch could step in, thus brushing what happened between them away so they could all focus on a new plan, enter Peeta?