Jacqui wrote:

I suspect that the coup referred to in 'Traitor' occurred before 'Terminal' - Servalan was intending to come back in the Liberator and destroy anybody who had plotted a coup against her

I think her position was under threat before Terminal - hence her desperation to secure the Liberator, or a new fleet c/o Moloch, though these frequent away trips probably helped to weaken her power base. But I don't think she had actually been deposed, because none of the Liberator crew mentions it. Her supposed death at Geddon was evidently public knowledge in Traitor; Avon and co would have had to be more than usually incompetent not to have noticed that their best-known enemy had just been overthrown. Cally was quick enough to tell Servalan she had no authority any more in Powerplay, when it was just a false alarm.

Therefore I assume that her enemies had been waiting for their moment, took advantage of her temporary absence after Terminal to start putting their plans into action, and were confirmed in power after a brief civil war concluded at Geddon.