I am by know means sure, but i think its a case of under serious g-force the seat collapses. I dont think the force will just lift the latch.
Imagine yourself sat their and there is no seat back at all, just you being held in by the harness. If you then had a head on crash your top half would still go forward but downwards as well because your shoulders would just burn on the strap and compress you downwards. Side fixings eliminates this.
I could easily be talking sh*t though.