Maybe Frog kicked his ass.
...ah maybe not, they do mention he summoned, or tried anyway. I assume he got killed, and Ozzie, Flea and Slash, who HADN'T been detained in the Lavos Timeline by Crono and crew, were probably out handling the army and whatnot. They come back...Magus is gone, inexplicably. Ozzie assumes the worst, giving Magus the benefit of the doubt, and praises Magus and himself.
In Keystone Timeline, all three assistants are detained by Crono and crew, however none are slain. When it seems evident that Magus WAS slain by Crono and crew, they turn to Ozzie as a new leader. I assume that he wasn't a very effective leader since the war still ended in Guardia's victory, whether or not Crono and crew completed the O-S-F sidequest, but enough to be praised as the final master of the Mystics in the war.
...*whoosh* or *applause*? lol