But Ellefson is the second most important band member of Megadeth, outside Dave he's the only one who has a semblance of being an actual "band member" rather than "hired gun who sooner or later will get the boot". If he fires the random drummer, "haha, classic Dave" and people move on, but to get rid of Ellefson he needs something more.
Or maybe this is a golden opportunity, he fires him just because he can.... Mustaine is an asshole, a tyrant, all of that. He fires him "with cause".... well, he's justified.