What would cause this to happen?

The Mad Hat Man said:

Hows that gonna work then - apart from filling the engine with water/foam/dry powder (take yo pick)?

Click to expand...

Meant I'd go for Wikipedia suggestion of the fire extinguisher:

Several ways to stop a runaway diesel engine are to block off the air intake, either physically using a cover or plug, or alternatively by directing a CO
2 fire extinguisher into the air intake to smother the engine.[3] Engines fitted with a decompressor can also be stopped by operating the decompressor, and in a vehicle with a manual transmission it is sometimes possible to stop the engine by engaging a high gear (i.e. 4th, 5th, 6th etc.), with foot brake & parking brake fully applied, and letting out the clutch quickly to slow the engine RPM to a stop, without moving the vehicle.

Wotever you do I reckon there's gonna be £££££ of damage

cover or plug... try getting close enough and a good enough seal on an engine that is essentially a hooge air pump.
CO2... as long as you can get/dare get close enough its viable
decompression... no, or very few modern engines have this facility, its the reserve of old air start/hand start old deisels, though I suppose a jakebrake might work on a larger truck
Clutch, if your quick enough I suppose, though I suspect a runaway is going to produce far more torque than a clutch could handle, the 2 runaways I know of (both non landy) just stripped the clutch when it was tried and mullered the engine anyway.