Musing on Buckbeak

[pippin_999]

No: HPFGUIDX 166232

I was thinking that it would have taken  a brave 
bureaucrat to pardon Bucky. If there had ever been another attack, 
that official would be held accountable.  I suppose Draco
denied that he'd ever insulted the thing. Hagrid couldn't
have heard him do it or he'd have put a stop to it. Harry's evidence,
if he'd been called to give it, wouldn't have done much good.
Everyone knows he's got it in for Draco.

 Then I remembered that Buckbeak did attack again.
He went at Snape, chasing him out of Hogwarts in HBP. Now, since
we all know Bucky is not a mad hippogriff and Snape hadn't been
insulting him, what made Buckbeak attack? Any ideas?

Pippin