This is a familiar story to me, because the same thing is happening with University of California admissions. But in California, it's clear that the number of in-state applicants is climbing fast. At UCLA, for example, there were 63,516 in-state applications for Fall 2017, compared to 43,938 in 2007. So the number of Californians applying to UCLA increased by 45% over the past 10 years (without including any non-resident applicants). UCLA has not been able to rapidly increase its capacity to match the rapid increase in demand, so the in-state admission rate has fallen. Nobody likes this, but at least it's understandable.
Is UM getting similarly flooded with in-state applications – in a state where the number of high school graduates is falling? If not, then the decreasing in-state acceptance rates over the past 10 years may be harder to explain.