Nice sidestep. You can't explain it because it's glaringly obvious Michigan was benefiting from them calling it very differently the other way. And literally a just few plays later when the progress was not nearly as securely stopped but defenders were swarming.
If a game is fairly called, things this glaring shouldn't me called differently.
Put a different way, when it comes to holding and PI, people usually take the approach of "as long as they call it consistently both ways, ot is what it is." Some officiating crews are lax on one or the other and let the teams play, and that's normal. Heck, I yelled about holds a few times tonight but they were pretty even handed on not calling them either way.
But on nearly identical crucial plays occurring very close to one another in game time, having two very different standards is inexcusable.