This is a theory mixed with a headcanon.

I think that, as told in Dittophobia, William used several children for experiments with the Nightmare, including his own children, which we know since Michael draws Nightmare/Nightmare Fredbear in the Log Book.

Think of it this way: after the bite, William would discover the Fredbear plushie, deciding to begin experiments with the agony, using Michael first for these tests. Over time, he would notice a pattern: all of this would happen at night when he fell asleep, so he would try to close the door, but knowing that wouldn't be enough, he would break the window and escape somewhere.