It's most likely as you say, needs to burn fuel the old way, so they're at 12,000ft for a few hours and then RTB. From the beeb….
13:30: Flight returns to Heathrow 'over maintenance issue' A US-bound United Airlines flight is returning to London's Heathrow airport following a maintenance issue, the airline has said.
Flight UA28 to Newark, New Jersey, had been circling off the south coast of England for about two hours.
It has 227 customers and a crew of 13 on board.