My gut feeling FWIW? Pilot error.
Mine too.
The Daily Mail states:
One of the cameras showed the aircraft was travelling at a 100knots while inverted at the top of the loop.
How unnerving! I've never flown a jet, but I opine that 100 knots would not be adequate airspeed to pull 1G into the loop recovery, let alone the several Gs required to recover the loop. So there would have to be a lot of trading altitude for airspeed on the way down - it seems there was not adequate room.
Also:
The AAIB said: 'The ground marks and photographic evidence show that the aircraft struck the road in a nose-high attitude ........... . The first ground contact was made by the lower portion of the jetpipe fairing,
High AoA all the way down the recovery of the loop creates drag, and reduces acceleration needed to achieve the speed to make G to enable recovery. High AoA still being held at ground strike.