It’s an old tradition, probably dating back to when Maine was part of Massachusetts and they didn’t defend us from the invading British in the war of 1812, esp. during the sacking of Bangor and Hampden. This fueled Maine becoming its own state after the war.

Maine gets swamped by Massachusetts tourists. I was on a rafting trip there. The guides made jokes about it to shit on MA (in good fun), and sure enough, 80% of the bus was from MA. I'm sure this is almost always the case. But I'm not sure people from MA are easy enough to identify vs. other northeastern states?

I think it has something to do with the number of people from Mass moving to Maine and retiring. Contributing very little to the economy, increasing the cost of real estate and bringing their political preferences with them.