Linda, don't want to be the bearer of bad news but, I would be concerned about a virus and or poison the squirrels are coming in contact with.
If the bodies are intact and seem to have just died, even rabies comes to mind.
There are so many new viruses. The west nile virus would do the same thing.
West nile is contracted by the mosquito and I am sure you know how rabies is spread.
I am not sure about the squirrels getting the west nile, we found dead birds around our place and took them to the state lab for testing.
