Sirius and the dementors

[jastrangfeld]

No: HPFGUIDX 45862

Ok.  With all this debate over the animagi . . .  (and I am not sure 
anyone will see my little post and I would love people's opinions 
here).

Sirius says he escaped the dementors in Azkaban because his emotions 
were less human, so he could slip by them.

So, when he's transformed into Padfoot, and chasing Scabbers out on 
the Hogwart's grounds . . . the 100 dementors come out and force him 
to change back to a human.  

My questions, how and why did they find out and force him?

Julie