Did someone make a promise for the release of the hostages?
It WAS very sudden if you remember.
As far as i know & throughout, Carter admin was negotiating directly & via his proxies for their release. President Reagan’s election helped but wasn’t the only factor. In fact, my take is that Pres Reagan was the only one who have dealt effectively with the mullahs but with the Soviet situation back then i’m guessing he had a bigger fish to fry.