Thanks to Steve for this info.

If when braking gently from 5 MPH to 0 MPH and the ABS starts to work, the problem could be with one or more of the ABS sensor rings. Jack the vehicle up and check the rings to see if any of them are broken. (you can tell easily as the gap will be longer on the broken one). Once found you will have to change the CV joint or the rear hub as necessary.

What happens is that the sensor reconises the wider gap and thinks the brake has locked up.