David, I assumed that is what you were going to find. The giveaway to me was no so much the flopping but you'll notice in your video that your yaw motor keeps moving back and forth. It was trying to pull away from something that was holding tension on it.
The flopping, in my opinion, is from these small motors being overloaded. This can happen from something rubbing up against the yaw motor (such as in your case), g-forces from aggressive flying or from wind. Lots of manufacturers write a shut off feature into the gimbal software to protect the gimbal in these cases, but the problem I see is that these motors are too small and too weak, hence, it takes hardly nothing for the floppy condition to happen.
The flopping, in my opinion, is from these small motors being overloaded. This can happen from something rubbing up against the yaw motor (such as in your case), g-forces from aggressive flying or from wind. Lots of manufacturers write a shut off feature into the gimbal software to protect the gimbal in these cases, but the problem I see is that these motors are too small and too weak, hence, it takes hardly nothing for the floppy condition to happen.