Agitated Crows

We're not sure what happened outside our home, but at the moment, a large host of crows is circling our back garden high up in the treetops and raising a cacophony. The frogs as well are making great croaking noises in their tiny bodies. Whatever has them agitated is out of our sight or understanding, and none of them are coming low to the ground to feed. The cats are terrified and came running back inside, but considering I was aware of where all three cats were at the time the crows started freaking out, I know it wasn't because of any of our felines' mischief.

What I find most strange is that we don't typically have a lot of crows around our house. But they're suddenly swarming and circling and cawing and ripping through the highest branches with alarming speed.

Perhaps they are being attacked from above? I have seen this sort of behavior when territory or nests are being threatened, but given our usual crow-lack on this portion of an acre lot, I can't imagine why they're acting defensive of it today.

Whatever it is, the frogs are nervous and the cats are fleeing.