In truth I do not know too much on M-ak. The name "the Ms" seems to be a subtle reference to ETA(m) (i.e. militar), once upon a time contrasted with ETA(pm) (political-military), which mostly dissolved in the 80s. The phrases the Ms (M-ak in Basque) and the PMs were no doubt used back then to mean either paramilitary organization.
However if you tell me it is for any other reason, like all the members having names beginning in M, I'll believe you. Still... it may have double meanings.
Most important about M-ak is that they made some kind of music that was surely a bit before its time, at least for Basque standards. Here there is an example: Barkatu ama (Sorry mum), maybe their best-known song:
There are some more videos in You Tube but I don't seem able to get them to play.