I think it's OK for Michigan to politely demand an answer. And, since that answer will almost assuredly not be "oh we're just systematically biased," it's also OK/smart to discuss a way to test or confirm the answer they do get.
For example, if the Big Ten says it's just outrageously bad luck/coincidence, then Michigan should expect reversion to the mean - maybe even quickly. If the Big Ten asks its refs and says it may be more due to something structural about Michigan's defense, then Michigan can go back and count holding penalties when they were/weren't doing that thing (to confirm).
Either way, nothing bad can come from going to the Big Ten and saying "hey, here's a weird/concerning observation; let's talk."