Mike left Blizzard some months before the layoffs were announced and the general conclusion was that Mike was told by Bobby Kotick that he had to do the layoffs or leave Blizzard, and hence Mike ended up resigning.
His resignation email said all the same stuff like "Mike wanting to spend more time with family" etc etc. But now it's fairly clear that was all crap, he still wanted to run a games company, he just wanted to be free from Activision and their general heavy handed management and manipulation of Blizzard.