Was the nature of Celes' betrayal ever discussed in the game? I though maybe she began expressing anti empire sentiments which lead to her being branded traitor but if that were the case, then she would probably be locked up in a prison in Vector as Kefka was. So either her change of heart came some time during the siege of Figaro (Ie the time it took to get from Figaro through MT Kolts to the Returners hideout) in which case what could have happened at these times to lead to her imprisonment (being general it is assumed either gehstal, kefka or leo had to be there to outrank her) or some series of events after her betrayal lead her to South Figaro.

THEORIES AND EXPLANATIONS PLEASE!