Why were they going to arrest Hagrid?

[garybec]

No: HPFGUIDX 98243

This is my burning question that I haven't seen addressed yet. 
(Although I'm new, I looked, sorry if I missed it.) On the night of 
the astronomy O.W.L., why were Umbridge and her cronies going to 
send Hagrid off to Azkaban instead of just giving him the 'sack' 
like Trelawny? The only explanation I can think of, is maybe 
because she thought he had put the nifflers in her office?  But is 
that a crime worth a term in Azkaban?  Maybe I missed something else?
I need some input please, and thank you!

Becki