I had never noticed the buss rails before, so had assumed BORG wasn't powered except in the station itself. This sounds much like the X-Flight / Batwing setup, except it doesn't go all the way up the lift like on those. Would it be correct to assume that the train would be powered enough to operate the seat release mechanism after it had left the station, but before it had hit the lift?
If that's the case, perhaps during the short interval between the train being dispatched and losing connection with the buss rail, it's possible to release the seat lock mechanism. That might also explain the mysterious seat release button that would seem somewhat odd (or stupid) to have to begin with, why it wasn't caught by some sort of safety interlock, and maybe why it hasn't happened before.
My thinking here is that if you hit the regular seat release button with the train in the station, it won't dispatch until the seats are locked again. If you hit it after the train has dispatched and has cleared the power rail, nothing happens as the train is unpowered. Hit it during the few seconds the train is rolling out of the station and still powered, though, THEN this button becomes the mystery Mid-Ride Seat Release button. Maybe this failure mode wasn't thought of, or any sensors to detect the seats not being locked didn't detect the problem after the ride had dispatched. Someone the other day just happened to hit the button at the wrong time and hence the incident. This is pure speculation of course, but I think this theory is somewhere in the ballpark.
Oddly, on the radio this morning they said Carowinds had added a buzzer to alert them of this problem in the future - I'm guessing it goes off whenever the button that was pressed to cause the problem is pushed. I also imagine that any logic flaws in the control programming / setup that let this slip by would also be corrected, but I'd guess it works better for them to just inform the media of the buzzer.
If you hear a buzzer after your train is dispatched, at least you can prepare for a more-interesting-than-usual BORG ride.
KenB