It happened to me once as well, albeit to Lusotannan and this was only a few turns into the game (at about 270bc).

What I think happened in my game was that the faction leader and heir were in the same army and they both died in battle (the leader has died, so the game makes the heir the new lea....there is no heir, faction destroyed!). Or, it could have been an aggressive rebel stack, for they left their capital practically undefended.

I'm guessing for your case, considering the size of the faction, it must have been the leader and heir being killed in the same battle.