Well . . . it's all a bit vague, but what is known for sure is that the English word comes from the Old French word mousseron (still used in modern French to refer to a type of wild mushroom), which in turn comes from the Latin mussirio.
It's because of the way they grow all clumped close together. This means you can grow them in really small spaces where there's not 'mushroom' for anything else.
I thank you.