Speculation...
BBC2 tonight had a number of "guessers" but wind shear ranked highly. Forecaster said just the right conditions existed at Heathrow at 12.45 for such an event and to hit shear with the aircraft carrying maximum drag must have caused the pilot to do everything possible to lift the nose over the boundary fence and get the aircraft down. Wet, soggy soil from days of rain would be devastating to the undercarriage which was clearly ripped off on one side and thrust through the wing from high G impact on the other.
Whatever the cause the Flight Deck did a great job and full marks to the Cabin Crew who reportedly started the evacuation without an audible Flight Deck instruction. First class conduct all round.