[hinckley buzzard]

the most likely explanation is that they were thrown to the ground by a severe downdraft during the storm.”

Windshear. It has dropped airliners like rocks before. Too bad, geese don't have the new doppler radar to warn them.

[tflabo]

Wonder if hail could have an effect....konking on the heads as a shock (concussion) and then they fall to the ground.
Just a guess...not an expert!