Curbed has the scoop:
BREAKING CurbedWire: Atlantic Yards Building Partial Collapse
Seems the parapet which was at the top of the building (duh) fell off following the start of demolition work on the building and heavy rains.
"Can the neighborhood look forward to this quality of work from Forest City Ratner for the next 20 - 40 years?"
An update from 1010WINS.com:
Brooklyn Building's Facade Collapses, 350 Residents Displaced
Photo from Curbed