Hi, I think you will find it is the sensor ring of one of the brake disks that has either split or has grease on it. If you are lucky the sensor ring may just need cleaning but usually you will find the sensor ring and the sensor need replacing sometimes with replacement of the drive shaft if it is on a driving axle due to the rubber boot having split and sprayed grease onto the sensor
A split in the ring or dirt on the ring or sensor sends false information back to the sensing device which thinks the brake is locking or slipping, this confuses it and brings on the ABS light which of course informs you of a fault on the braking system
I hope this helps