Initial contact was actually Pettigrew grabbing the facemask. Watch the from behind angle, Pettigrew grabs the facemask and you can see the helmet getting tugged left and right prior to him letting go to reach for the ball. From the LOS you can't even see him grabbing the facemask and it looks like the defender pushes on his right shoulder causing him to fall. From behind you can see the defender getting tugged around and actually it doesn't even look like contact was even made, it looks like momentum carried Pettigrew because he didn't plant to come back to the ball.
Gotta remember officials don't see the game from multiple angles and in slow motion. Back judge would have seen no penalty/penalty on DET whereas any official near the LOS would have seen for sure PI. Who knows what it would have looked like to a side judge.
http://www.foxsports.com/nfl/story/...ens-brandon-pettigrew-overturning-call-010415
If you want to blame the incident on anything, blame the NFL. It sure sounds like the refs were under pressure to use their brand spankin new headsets to communicate and not to huddle up on plays.