Actually, it's not that the Keyblade Wielders started attacking him, it's more like they had to fight because it was the final battle. Noone knows (well, noone here) who started the real fight.
That may be but the only evidence we have points to the apprentices, because Terra had already beaten up some kind of creature when he arrived at the field, just before Aqua and Ventus arrived. Then MX appeared from way off in the distance and Terra led a charge on him, almost everything MX was doing was reactive.