This sort of mass die-off is not unusual in nature, any more than unexpected "blooms" of plants and animals. It is usually due to some local event, perhaps even a perfectly natural one, that kills the animals, or poisons their food, or something else of that nature. The thing to watch out for is if it starts happening more and more frequently, that is, watch out for a long term trend, not just a dramatic incident. Even trends can be misleading, because we are much more attuned to this sort of news than we used to be. It is reported more often and the cameras can get there faster.

This doesn't mean that there is nothing sinister going on, just that we don't know for sure. Maybe it's just nature's way of telling you something's wrong.