The predator theory looks favourite to me, they flock together when threatened, safety in numbers, looks like the flock just dived into the ground
as an aside I was once in Scotland at dusk in the summer, as I drove down the road I could see something moving fast about a mile away, as I got nearer I realised it was a huge flock of birds above a small copse of trees.
No exaggeration there must have been 500,000 of them, the sky was black, they wheeled around for about 10 minutes then it was like one of then had said NOW, within seconds they all landed in the trees.
the noise was unbelievable with them all squawking, it was one of the most fascinating things I've ever witnessed