RE: My radio cut out when the motor cut out. What happened?
In regards to the power problem, it sounds like the ESC is working as advertised. You want it to cut out at 9.0 Volts when you're running a 3S (11.1V) LiPoly. That's 3.0 Volts per cell. Run it any lower, and you will cause damage to the pack.
When a pack's voltage drops drastically like that, it means you're drawing more current than the battery can handle. Since you have another identical pack that's handling it fine, I would first suspect that the pack that cut out might be a bit weak, possibly damaged, possibly bad from the factory. Call ThunderPower customer support on that one.
As far as your glitch... I don't know if anyone can say for sure what the issue is. You might want to send the ESC back to Castle, and the receiver back to its manufacturer to be checked out. It may be that the BEC truly "kicked in" when you hit the low voltage. You want the LVC to kick in, not the BEC. The BEC powers the receiver, hopefully all the time, while the LVC cuts off the motor when the low voltage limit is reached.
You did say that you were running at a high power level. This is just a theory, but perhaps the heat buildup was an issue?