Now, the birds shown are Red-winged Blackbirds and Common Grackles -- common species to find along side non-native European Starlings in big flocks of "blackbirds". From the condition and seemingly localized occurrence of the bird carcasses shown in the video, I'm pretty comfortable ruling out traffic or power-line collisions as the source of all these dead birds. I'd be much more included to put my money on poisoning... and probably deliberate poisoning.
Why? Well, this sort of thing actually isn't all that unheard of... Take, for example, a case from New Jersey which previously made the news when birds started actually falling from the sky.
[Source: WKRG.com News]
Oh, and if you wondered what was meant by "a permit" in the first video above, I should mention that the New Jersey incident (and this one from PA) were both part of control efforts. While hardly a pleasant sight, these large flocks of (mostly?) European Starlings were poisoned after consideration and approval by the USDA.
Getting back to the original question of "what happened?" we (fortunately) won't need to rely solely on speculation in this case! That said, we do still have to wait a bit and see what information comes back with those lab results...
(To be continued!)