My daughter locks the seatbelt all the time in my car. There is a mechanism that locks the belt reel when it detects that the belt is being accelerated. This is suppose to prevent the belt from feeding out during an accident. It can also activate if you pull fast on the belt. The only way to release it is to feed it all the way back in.

I suspect that the middle belt is looped over the bottom cushion corner and when you pulled up on the seat back quickly it locked. If you can unloop the belt and feed it back in it should unlock.

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Wake up every day that would be a start.