It does not appear to me that the birds dropped from the sky. It seems like, for whatever reason, they flew under their own power directly into the ground and surrounding scenery.
A reddit comment I saw last week mentioned it could be just the flock/murmuration were trying to get away from a predator and flew lower, and the ones at the bottom didn't have much luck and hit the ground. Would at least make sense...
Ornithologist and biology professor at Allegheny College in Pennsylvania, Ronald L. Mumme told Newsweek he thought a large bird was behind it.
"Based on the birds' behavior, my suspicion is that a falcon or other bird-eating raptor attacked the flock in flight, and during evasive maneuvers the flock members dove, became disoriented, and many crashed to the ground and injured themselves," he said. "I don't think any birds were having problems until they collided with the ground or buildings. It looks like most of the birds were able to fly away, but a significant number could not."
Two more experts told The Guardian they agreed with Mumme, blaming the event on "flock murmurating to avoid a predator raptor."