Maybe a microburst [0] of air from a developing thunderstorm? Basically, an extremely powerful downward burst of cold air. Seems to have killed birds in the past [1]. This is absolutely terrifying during takeoff/landing inside an airplane [2].
The article from The Guardian quotes as the most likely cause:
"Dr Richard Broughton, an ecologist [..] said a predator could have made the birds swirl tightly and driven them towards the ground, with higher birds forcing lower ones to crash into the buildings or the ground."
"Dr Richard Broughton, an ecologist [..] said a predator could have made the birds swirl tightly and driven them towards the ground, with higher birds forcing lower ones to crash into the buildings or the ground."