Throttle - Both the pitch/roll grip and the throttle are fully digital, with zero mechanical links to the parts they control. This means that their function can be changed during flight, which is what the F-35B does. When the F-35B is in "Mode 4" (using the lift fan) the throttle no longer controls the thrust output of the engine (unlike a Harrier) but now controls the fore/aft movement of the aircraft. To do this the throttle is set in the middle position and then fore/aft movement of throttle moves vanes at the bottom of the lift fan and moves the engine nozzle to direct thrust to move the aircraft forward/backward. The thrust (height) itself is then controlled by what was previously the Pitch function of the right side control column.
The basic thought process behind this design choice by LM is that pilots understand the throttle makes them go forwards faster/slower and the Pitch makes then go up/down during normal flying so lets keep the same setup during the hover as well. As this is a fully FBW aircraft with no mechanical links its a simple software command to slowly blend the change of throttle/stick functions between flight and hover when the pilot commands a Mode change into Mode 4.
Engine Action during Incident - The reason I point out those previous things is to help explain what happened after the nose dropped (still only a guess why that happened). Once the nose dropped the engine can be heard at full throttle still but the aircraft doesn't try to rise. As mentioned this due to the throttle no longer controlling height but fore/aft movement, which is why the aircraft continuously tries to move forwards but with both a limited amount of thrust (20KN) and only a limited amount of nozzle deflection the amount it can push the aircraft forwards against the friction of the ground is fairly limited. My pure guess is that the pilot, with their hands on both sticks, due to the impact of the nose dropping onto the floor, pushed the throttle forward as part of the impact as they were slammed forward, but again that is just a guess. Then post nose drop the pilot is just holding onto a bucking bronco of an aircraft and I guess they probably had their hands already on the ejection seat handles ready to eject, rather than trying to move the throttle back to a more neutral position or to kill the engine completely (again, just a guess).