Full details in the Haddon Cave report [0], the official investigation into the crash of Nimrod MR2 aircraft XV230 in Afghanistan in 2006. The 'simply' here (as in 'simply' caught fire) was in fact a catalogue of known problems and missed opportunities. The Nimrod variant involved was originally intended to perform long-duration maritime surveillance and was designed to allow the crew to intentionally switch off one of the engines when the plane arrived on station (to save fuel while loitering for hours). To restart the engine there was a pipe routed through the fuselage that carried hot gases from the running engine to kick start the cold one.

A separate modification of the complicated tanks and plumbing that supported in-air refuelling led to a (leaky) fuel pipe that was routed right past the (very hot) engine cross-coupling pipe. Very minor leaks somewhere in the system were routine.

In this case there was a fuel leak near the engine cross-coupling pipe. The unfortunate crew of 14 had six minutes from initial fire warning to the eventual and unavoidable explosion that killed them all.

[0] https://www.gov.uk/government/publications/the-nimrod-review