Why did British Airways flight fly in loops over UK after suddenly dropping 28,000 feet?

A British Airways London-Chicago flight, which declared an emergency over Ireland on Monday after descending around 28,000 feet within minutes, turned back towards London and flew holding patterns. As the aircraft was carrying fuel for the transatlantic journey, the Boeing 777-200ER burnt fuel by flying in loops to bring its weight closer to appropriate landing weight before landing at Heathrow.