Something started by Gunhaver, who currently runs opethforum.com. It wasn't meant to piss anyone off, but he might say differently.
He started making sigs that were jokes about the band. He made one that had all the members pictures with their name spelled incorrectly, as most fanboys dont know anything about the band. Instead of putting Martin Mendez, he put "bass player" as a little joke. Well it stuck and now people are chanting it at shows. We all love Mendez, hes such a great musician, but its okay to joke once in a while.
Haver said that a band played at his school once, and all the members were from his school but the bass player. So everyone called him "bass player."
There you have it. Hope that clears it up.
See also: "Half past Bass player"