What the hell!?!

Just wondered, does anybody know why the plastic shop manikin came to life in the movie? I know the film hints at some *beep* that due to all the art students making the thing they each add part of themselves to Jigsaw, but surely that can't be the reason why it came back to life!?! I was hoping for at least some kind of back story, like the manikin was in a store where someone was murdered or involved in some kind of satanic ritual, but nothing like that.

"Welcome to your doom!"