Squirrelly Challenge

Here's how Merriam Webster's online dictionary uses Squirrelly in a sentence:

It was the end of the school year, when all the kids get a little squirrelly

It's easy to find what the word means. But what causes squirrels to behave this way?

Help us build this search challenge. Send the URL of an authoritative article or document that provides a clue to wizard@21cif.com