It either means that
1. There are a lot of people betting on Michigan or
2. Vegas "knows" something that the public doesn't or they just feel really strongly that Michigan will win regardless of what people are actually betting
The second scenario is why Lax brought this up and if you believe that games can be fixed, this situation is kind of disconcerting.
Yeah, and games can/are fixed. Regularly. It's incredibly common internationally but also pretty prevalent here in the USA. Usually, it's the smaller games though and not the marquis matchups for a couple of reasons but I'll save that for another day.
I just talked to a guy who is a professional gambler. He doesn't do it by having "ins" or being a "shark" or using the tactics your typical syndicate uses... rather, he wrote computer programs to play lines off each other online when one book is giving starkly different odds than another book or there are other inefficiencies in the system.
Anywho... he says that the problem is that the public money came in really hard on Michigan. First at -3 in a landslide, then -3.5... and it didn't flip back hard at -4.5. He said there is like 65%-70% public money on Michigan and that's likely why the line just jumped to -6 for some books is that they're trying to even up their money so they don't take a bath if Michigan wins. He said, as far as he is aware, the "shark" money is actually leaning on ND.
So on the surface... doesn't seem like there is anything to be worried about. Still, I'd keep an eye on the refs... just in case.