Back when the east side of this country was continuous forest, there were massive grey squirrel migrations. Imagine a million squirrels all going somewhere. In the trees, on the ground, making a racket like we can't fathom. I think I may of read about one such migration in the 1700's in a book about the early history of Walton county.
There have been large scale grey squirrel migrations recorded as recently as 1968. Google it.
So basically what causes this is an incredible acorn crop followed by an easy winter with low amounts of predators, they breed to great success, there might be a number of years of great acorn crops that sustained them and easy winters that didn't kill many. But then the trees fail to make acorns, this large population of squirrels is now starving and the only thing to do is find somewhere the acorns did great. A hungry squirrel, thousands to millions of them will cover some ground.