How, exactly, do bicyclists "endanger" motorists?

one of the many articles had this snippet:

Deputy Police Commissioner Paul Browne said the bikers had caused ''massive disruptions'' and endangered motorists.

While it is true there were massive disruptions, I still have not come across an article that explains how 200 lbs. of man and metal endagered a couple of tons of metal last night. He did add:

"They were blocking intersections all along the way, backing up traffic," said top NYPD spokesman Paul Browne. "I personally witnessed several ambulances that couldn't get through. They had their lights and sirens on."

So how is it that the police were able to barricade certain routes, yet they were not capable of stopping the crowd long enough to allow emergency vehicles through?!