My thought is that the system is designed to keep you stable while moving, and the possibility that you are trying to get un-stuck was overlooked. I have never tested mine under those conditions, so I can't say what it does. You could try a similar test while moving to see if the system activaes without errors.
My thought is that the sensors are OK because I think you would get ABS faults if they were bad.
Sky NA 2007 Midnight Blue
Sky RL 2008 Carbon Flash - Naked