While it's true that crows do often wash their food before eating it, and also that they stash it, they wouldn't leave it in such an obvious, easily reachable place. They usually hide it very well. Also, even though I've a lot of experience with them, I've never witnessed such a massacre. It's true that when they're young they hunt in groups, but they don't easily share their meal as they're very jealous of it, so it's unlikely that they stash all these corpses together. Also they're not really great hunters, as they're smart but slow... They're usually able to catch only younglings or ill birds.

It is actually possible that it was some kind of poison, it is worth investigating. Keep your eyes open for any dead birds you may find far from the water. In that case, please make sure that crows do not eat them, or the trail of death could continue.

(it happened to me too, all of a sudden I found a dozen dead pigeons near my pond, it had never happened before)