Draco
They're doing it to create unrest and chaos. They want people to turn on each other, so that they're distracted. It's a sleight of hand.
The question is, why was Sirius so eager to give into these demands when he refused every other time? Where are his Gryffindor principles now? It doesn't feel right. Either he's in on it, he's bewitched, or he's planning something fucking awful in retaliation. What pisses me off, is that he knows more than he's letting on, but won't tell the DMLE fuck all.
But wait - you're Draco fucking Malfoy. Shouldn't YOU know what's going on?