Hmmm, I'm not sure, honestly.
I've been running around with my brake/abs light on for over 3yrs, sensor went bad 4 times in less then 3yrs and I just said forget it.
If it was doing it ALL of the time, I'd say could be your calipers hanging up, and or e-brake stuck, but rolls fine in neutral, so I'm at a loss.
Hopefully someone else can chime in.
You have 4-wheel disc brakes??