Everyone knew. For a long time.
Meredith Whitney said so - point blank - on the TV show "60 Minutes" in 2010. The timing is a bit off. It happened in 2013 and not 2011.
Why? probably because she warned and people took short term evasive action; but cannot prevent the inevitable.
Also perhaps due to propping up of the cities during the 2012 US presidential election campaign.
Meredith Whitney predicted the subprime mortgage crisis. That made her famous.
In a 500 page report, she predicted the collapse of municipal bonds. It is in writing.