I really hate to put it this way, but I think what happened is that Peters heard footsteps. His pocket presence has been...sporadic at best. He does his best work on rollouts, but that also takes away half of the field. I think this is a problem with Sitkowski too; they both seem to overanticipate pressure. That's really been one of Peters' biggest issues: the happy feet, the lack of pocket awareness. These have really limited his great physical tools, and they're really difficult habits to get rid of in a hurry.
I don't have a solution to this. Sitkowski does the same thing and nobody else on the roster is physically the kind of QB Petersen wants to play.