About the only way to lose the nut engagement with a ball shaft is to lose the balls out of the nut.
These balls recirculate as the shaft turns, and anything that damages the recirculation pathway has the potential to cause disengagement of the nut from the ball drive shaft.
How could a recirculation pathway be damaged? There is always the potential for FOD in an aircraft. The right foreign object in the correct place and damage can happen despite the best efforts of the design engineers and maintainance engineers.
If the balls are missing from the previously pictured stabilizer actuator nut, the nut would be free to slide, and any position we see it presently would be coincidental.
Should the NTSB held FDR data shows a step jump in stabilizer position, they will be looking closely at this unit to identify the cause. But for now, it needs to sit untouched so as to preserve any potential clues as to what might have happened.
For those who have poor understanding of how a ballscrew system operates, here is an illustration: