It was indeed the collapse of Detroit. The population has been falling for decades since its peak of 1.85 million in 1950. The last decade was particularly bad, with Detroit dropping from 951,270 in 2000 to 713,777 in 2010. In the past, the metro area would still gain (or at least remain steady) even with a declining population within city limits. That wasn't even the case from 2000-2010.
Chicago suffered a similar drop during the decade of the 2000's losing 200K residents. Chicago is a lot bigger than Detroit so the drop was a smaller percentage, but Chicago's loss was primarily in the black neighborhoods and those are roughly the same population as Detroit. The Chicago drop was steep enough to cause Cook county to also lose population, but some of the suburban counties were among the fastest growing in the nation (Kendall, Will) during the decade, so the state as a whole gained.
