TMZ Sports broke the story ... Mason got the ax from BIG3 on Monday for refusing to cooperate with a lawsuit against two Qatari investors who allegedly screwed the league out of millions of dollars.
BIG3 says Mason has a personal relationship with the investors -- that he may have taken gifts from them -- and he essentially chose siding with his friends over effectively doing his job as commish.
In fact, after BIG3 sued the investors, Mason commented on an Instagram photo featuring him with the two men saying, "#family."
But Mason is calling BS and claims the real problems center around BIG3 co-founder Jeff Kwatinetz -- who has "made countless unfounded attacks on my integrity, character, and leadership."
Mason suggests Kwatinetz targeted him, in part, because of his race.
"The work environment at BIG3 has been hostile and racist resulting in the departure of valuable League personnel. Among other matters, a former employee of BIG3 recently told me that Kwatinetz has repeatedly referred to black athletes as 'rich n**gers.'"
Mason says he's "disappointed" that other BIG3 execs -- including Ice Cube (who's black) -- would join with Kwatinetz in "leveling these desperate manufactured claims against me."
A spokesperson for Kwatinetz and Ice Cube tells TMZ Sports, "Mason's claims do not require a dignified response beyond the fact that they are preposterous and false."