Ooh! The plot thickens! Whatever it is, I'm willing to bet that a Pink Pony Lunchbox is somehow the ultimate root of it all. A kid can hold a grudge for the weirdest stuff you know. I think that we are going to learn the answer as to what started this very soon.
A prediction: Either Kat and Hilary are going to end up in a tearful reconciliation and end up closer than sisters or they are going to end up accusing each other of "starting it" and getting into a cat-bunnyfight (again). In the latter case, it's even money whether Pip would intervene or just record in both still and video forms for later upload to the appropriate sites.
One way or the other, though, I think we'll never look at a certain bunnygirl in the same way ever again!