Front Passenger Seat Unexpected Recline

'99XC70; During a long road trip this past weekend, the front passenger seat-back appears to have "slipped" into a semi reclining position after the car traveled over a long bump in the road (slightly elevated railroad crossing). This happened instantly, not gradually as would be expected by operating the electric seat control to recline the seat-back. Its like something in the mechanism suddenly "slipped". It was not a heavy person sitting in the seat. The car has 180K miles on it and to my knowledge this has never happened in the past. This happened twice on this trip, however both times, we were able to raise the seat from the semi-reclined position to the normal position using the electric controls.

A relative of mine had later XC70 (mid-late 2000s?) and was involved in a heavy rear-end collision, When the collision occurred, the driver's seat appeared to recline upon impact which I presumed was a safety feature to protect against whiplash etc. I am wondering whether this "controlled recline" is designed into these cars? and if so, does this mechanism require servicing.

Has anyone else experienced this? Searched the forum but was unable to find anything.

TW