There is no 'official' reason yet stated however thre has been much discussion on the official forums. Most think either it was the result of dark side corruption or Revan cut it off.
Some others are:
1: he was learning to twirl his lightsaber - that musta taken a lot of practice, and a few false starts.
2: he lost it in a fight with a LS Jedi, some time after he went DS.
3: he did something to seriously annoy Revan, and that was his punishment.
4: something during the Mandalorian wars? Maybe he tried to fight Mandalore himself, that happened, then Revan finished the fight for him?
5: he tried to kiss HK-47.
6: I'm running out of ideas here, but maybe a Gizka did it?
7: He saw Bastila nude and his jaw just dropped to no repair. (Note: I didn't think any fo these up)
8: Shaving accident
See below for mor information:
"Malaks jaw" "Implications of jawlessness" "Malak's Jaw?" "Malak's missing jaw" "malak's jaw?' Links for the rest of them