The hydraulic lines from the engine route through the back of the pylon up onto the rear of the rear spar.
So I'm guessing a considerable internal pylon / front spar main fuel supply fuel leak flooding the pylon with fuel therefore allowing vapor to flood the leading and trailing edge cavities during the extended diversion.
Airflow prevents pylon fuel / vapor from contacting hot engine till roll out, kicking off the fire.
Engine change without new fuel line seals or incorrectly connected fuel line etc during maintenance.
Any maintenance carried out recently?