Angel knows the reason. Rose was bitten by a werewolf and is suffering the degenerating side effects as the bite starts to kill her and drive her mad as it does so. He's not going to tell her that though, regardless of whether he feels Quinn deserves the truth or not.

"Glitches. It might not have even been someone who's normally a vampire. Instinct can be overwhelming, especially when you're new. You don't think, you just do."