Sounds like the war was entered when a defence pact was triggered (someone has attacked an ally of yours). I am not that good how it works (I rarely use defence pacts) but since the original war was between the attacking AI and your defence pact ally if they end the war your help is no longer needed and the also your Casus Belli for the war is removed when that happened.
As I said I don't use defence pacts so often (to many AI:s that starts to dislike you for the benefit of an AI that usually don't come to your aid anyway) so if this was the case, then someone who are better than me regarding how defence pacts work can enlighten us.
Edit: And as The Quasar said, the AI usually try to make peace with you, usually by giving you a whole bunch of crappy costly cities besides money and luxuries. But it is never forced. The only time I guess the war ends without your permission is as I said, when you have entered the war due to a defence pact and the AI you came to help is declaring peace.
