An alternative explanation I got from a FA (after having the headrest pushed firmly down on my 6-5 frame whilst asleep) was that in the event of an accident that leaves the plane upside down, the extra clearance between the seat back and ceiling (now floor) would be critical for people getting out on their hands and knees if necessary. At least it was original!

And, I've taken to helping to lower the headrest for folks sitting in front of me, when they seem to be struggling a bit to get their arms in position to do so themselves. Even helped Patrick Leahy do that a few months ago on a flight from DCA-ORD.