Main Fuel/Oil Heat Exchanger failure (Known fault on GE engines)
Higher presssure fuel enters the oil system and circulates around the oil system - hence the oil system parameters changing.
Fuel/oil vapour/mist vents out the central vent system and to atmosphere.
Due to cold stream airflow/forward speed of the aircraft nothing happens.
Flaps down on approach and lowering airspeed, venting fuel/oil vapour impinges on the lower wing surface.
Selecting reverse thrust plus decaying forward speed due to landing, no more airflow to draw the vapour away.
Engine central vent system vapour ignites and spreads to the contaminated lower surface.
Even after engine shutdown, system still pressurised and venting adding to problem.