How exactly does this happen?

From NBC:

Eight families have lost their homes after an apartment building in the Bronx partially collapsed, sending several tons of bricks cascading onto the ground.

Chopper 4 was overhead as firefighters responded to the Fteley Avenue building Soundview Thursday evening, with bricks and debris strewn on the sidewalk and on the street. Bricks fell with enough force to mangle the front gate and cave in the awning, with some hanging precariously over the entrance.

No one was hurt in the collapse, and the cause is under investigation. FDNY officials on scene said an occupant started some repair work but "it just looked like the age of the building, maybe weather."