Michigan was still a Republican-leaning state at that point. It had been very Republican early in the century, with a lot of the population being in small towns from Republican-friendly ancestry (New England Yankee, German, Dutch, etc.). But it had been gradually becoming less Republican, and this continued to wins by JFK and Humphrey in the 1960s.
The African-American vote becoming ~90% Democratic after the 1964 CRA almost certainly helped a lot in Michigan.
Yes, though since that vote was already Democratic in most places by the 1940s, the bigger difference there is the increase as a % of the Michigan population over the course of the Great Migration.
