>>2 while pollution might make dopiness avoid certain areas, it wouldn't make them suicidal, plus they can always just suffocated themselves (ask Flipper). We do know (thanks to the Japanese fishermen) that underwater sound can force pools of dolphins to come up really close to the shore, but they hardly ever wash ashore. I would venture guess that it might have something to do with the magnetic polar shifts, and that, also, if the OP could specify in what area this happened, because I am aware of a large spot off the coast of Brasil where Magnetic south has turned into North, but there are a lot of such spots all over the world.
And yes, flocks of birds are nothing, you should have seen the area I grew up in.