I have seen the report on TV, and if you look at where the birds are, they are confined to a very small area. I would think that this was a flock of birds that ran into something that killed them. I would agree with fireworks being the culprit. Might have even been a freak electrical thing, as those birds like to hang out on power lines in mass numbers.
I was once on a lake with a group of guys, and they dropped an m-80 into the water, and it exploded, and you should have seen all the dead fish that floated to the top of the water from the shock. Some were just knocked out, but most were dead. I thought it was pretty stupid of them to do that. I said, how would you like to have someone drop a bomb on you, just for fun...jerks.
They later set an entire hillside on fire with their stupid m-80s. The fire was headed straight up to one of their houses. We went and got a bunch of wet blankets and laid them in the grass leading up to the house, and the fire died. But, I would think that it burned about 4 acres of brush in like 5 minutes. So, you have to be really careful with that stuff.
Another result of the birds deaths could have been smoke from the fireworks, they might have died from inhaling it.