The most credible explanation about the Michigan polling debacle that I read was by the Votemaster at electoral-vote.com.
http://www.electoral-vote.com/evp2016/Pres/Maps/Mar10.html#item-8The polls for the Michigan Republican primary were spot on, but those for the Democratic primary showed Hillary Clinton winning by 20 points. She lost by 1.6%. Why? The Washington Post has figured it out. Short answer: The likely voter screen was botched on account of 2008.
Long answer: It goes back to 2008. In that year, Michigan violated the rules and, like Florida, moved its primary to January. Then-chairman of the DNC, Howard Dean, announced that as punishment for violating the rules, Michigan's delegates wouldn't be seated. Barack Obama supported Dean and didn't file to be on the ballot. Hillary Clinton filed and, naturally, won although only 600,000 people voted since voters knew the results didn't matter. This year (and every year), the pollsters try to figure out who is likely to vote, the so-called "likely voter screen." One of the questions they ask is: Did you vote in the last contested Democratic primary? That was 2008 and was anomalous because the DNC said it wouldn't count. Thus the pollsters were way off in guessing who would vote.As long as the 3/15 states didn't have any similar problems then there is no reason for pollsters to do the same mistake again.
Granted, Florida was punished too in 2008. But Obama and Edwards were on the ballot and their voters had the chance to go and pull the lever for them, and most of them they did.
