I was just getting off work when this was happening. I walked around the seemingly affected area and observed the following streets blocked off by cops (though perhaps there were more?)...
The traffic was terrible. It's times like that when I'm glad I don't drive... although there were a ton of buses stuck in the traffic too. I was surprised to see people still sitting in them considering they were going nowhere fast.
I saw a guy talking to a cop from a distance and then by the time I caught up to him he was talking on his cell and telling somebody that they closed the roads because glass was falling from some of the construction sites. When I took a look at the Flatiron it looked like there were some large glass panels missing from where I would otherwise think there should be some at this point in construction, though they might just have not put them up yet. Either way the streets around the Triangle West construction sites and the Shangri-La were closed off.