Could it be that he felt that his side would lose the war? Is there anything in the movie that made such an outcome very likely?
Yes you got it. Landa realized that Germany was going to lose the war sooner or later. The movie's main action starts to happen in June 1944, which probably means that the Normandy Landings have happened. Landa, being a very smart man, knew that since the vast American war machine has made landfall in Europe it was only a matter of time until they reached Germany. He also knew that if this happens he will be prosecuted as war criminal for his crimes and hanged. So, his solution was to help the Allies get an even earlier victory in exchange for safe passage for himself.