I'd read something about this somewhere before and If I'm right it works sort of like a one way pendulum triggered by G-force to move forward and lock and reset by unlatching the belt to move back.
Too much momentum from say slamming on the brakes or quick starts and stops, fast take offs, will lock it into place.
Might be the way you drive. Might be a loose seat belt mechanism.
You're right about the general way old-style locks work. The way mechanical seat belt locks work isn't a "one-way" pendulum, though. It's a pendulum that hangs straight down normally, then any force in any direction will make it swing to set the lock. This system can't work if the mechanism is inside a reclining seat back.
Many newer system don't have a pendulum at all. My daughter's Chrysler Sebring has electronic seat belt locks that are controlled by a g-force sensor under the dash. With our Tahoe's all-electric, reclining seats, with the shoulder strap in the seat instead of on the door pillar, I wouldn't be surprised that it's either electronic or something more elaborate than a simple pendulum.
Also, if it was from driving style, then the driver's belt should lock at the same time, too. It doesn't.
I'm gonna have to break down and buy the official Chevy shop manual, I guess.
