Well the UK guys (+ gal) were "kidnapped" exactly in the Iranian new year's celebration time span => lot of the political people were on vacation.
It was the "reformal garde" that kidnapped them (the very very hardliner). Iranian's president is one of these guys (not the garde, but a hardliner - against US etc.).
Now all the politicians are back from vacation and internally (from the inside of Iran) a lot of people (politicians) were pressing on Ahmadinedschad. Now he had to release the hostages...
The pressure was enormous (from internally). There is a lot people in Iran that doesn't like a war with the rest of the world. They would like to have stable relations with the other world. It's just the hardliners that don't care.
... that's how most news stations and so called "experts" explained it here.