My JAOO (now GOTO) SAS flight home from was slightly delayed. Over the tannoy, whilst we were still on the ground at Copenhagen, the captain explained it was because of early morning fog over Heathrow. He further explained that when its foggy air traffic control increases the gap between the planes in the air as they come in to land. This reduces the capacity and so to prevent a backlog of planes circling overhead they send out delays to planes so they take off a bit later. What this amounts to is that planes leaving can't take off until they receive a pull request from where they're due to land.