You have a sensor count issue. At slow speeds the sensor output is of lower voltage and thus the pulses can be miss read. This tends to become issues on elder cars as the sensor performance drops, or/and those cars with the following.....
Check for:
1. Rust on toothed ring (the part the sensor detects)
2. Mud on sensors
3. Poor ABS sensor electrical connections - this normally manifests as permanent ABS light.
4. Toothed ring cracks! (In my case)
This is a sore subject with me as the repair cost $1000! See here for more info.
ABS coming on - over sensitive!