I've experienced a few times recently where cabin crew have asked passengers to lower their adjustable headrests during takeoff and landing and, in at least one case, it was even part of the pre-recorded safety briefing.
I'm wondering what on Earth would be the reason for this?
I completely understand needing to keep seatbacks and tray tables upright and aisle armrests lowered, but having headrests lowered makes no sense to me. As a tall person, having the headrest lowered is uncomfortable. For extended durations (e.g. a long wait to takeoff, etc.,) it becomes extremely uncomfortable.
Even more concerningly, though, it seems like this would significantly reduce safety in the event of a crash, due to forcing a tall passenger's spine into an unnaturally-forward position and having neither most of the spine nor the head supported. It seems like this would be much more likely to cause head/neck/spine injuries to the passenger during a crash than would allowing passengers to adjust their headrests to properly support their heads and necks (and not push their shoulders forward.)
I see that 14 CFR 121.311(e) requires seatbacks to be in the upright position during takeoff and landing, but I see nothing in the FARs about requiring adjustable headrests to be lowered for takeoff and landing during Part 121 operations, though I may have missed something.
So, my question is what is the reason for this requirement for headrests to be lowered? Is there some regulatory requirement for this or is it an airline-specific thing?