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

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."