[svandamme]

from what I read on the design, there has to be a clutch to disconnect the lift fan from the engine turbine shaft..
If clutch broke
actuator failed
or oil leak on the clutch (if at all that makes sense for a F35 clutch vs basic car clutch )
electronic glitch disabled it

It sounds unlikely that the pilot could mistakenly just turn off that lift fan, while in low forward velocity hover... So I Doubt this was pilot initiated input.

But obviously the mishap report will look into it with more data then we get here


EDIT
"The clutch mechanism uses dry plate carbon–carbon technology originally derived from aircraft brakes. Friction is only used to engage the lift fan at low engine speeds. A mechanical lock-up is engaged before increasing to full power.[24]"
apparantly, it is supposed to lock up and can't slip after engaging..


"The gearbox has to be able to operate with interruptions to its oil supply of up to a minute while transferring full power through 90 degrees to the LiftFan."
maybe ran out of time on the oil supply?