Disc brakes do have a tendency to get locked due to higher efficiency level of braking of the system. While in motion when the disc gets locked, the wheel starts to skid on the road disbalancing the rider. Thus tangential resultant forces comes into effect on the whole motorcycle frame. Ultimately if this tangential force is not stopped in time, it forces the whole motorcycle to slide sidewise causing the riders to loose complete control of the vehicle & fall and may cause a major accident.
ABS on the other hand acts in such a way so as to stop the wheel but not completely locking & stopping the wheel. During a braking action the rider/s moves forward, thus moving the vehicle centre of gravity towards the front end of the vehicle. Thus stopping of the vehicle with the help of front wheel is more effective than stopping of the rear wheel/s.
Considering the vehicle is running at 90 Degrees angle with the road while brake is applied, if the front wheel is locked while the rear wheel is still rotating, there is a chance of vehicle getting overturned frontways due to shift of centre of gravity (towards front) and high inertia of motion and less load on the back side of the vehicle compared to the forward forces acting on the vehicle.
Considering the same above conditions (i.e. brake is applied & front brake got locked while vehicle in motion) when the vehicle is not running at 90 degrees angle with the road i.e. the vehicle frame is having an angle less than 90 degrees with the road (in either left or right side), then a radial force will act upon the rear wheel where the radious will be the length between the centre of gravity of the vehicle at that time to the point of contact of the rear wheel with the road. This radial force will try to move the rear wheel left or right – sideways (depending upon the < 90 degrees angle the vehicle frame made with the road surface during that time – either left side of the vehicle or the right side of the vehicle). Thus the rider will find (if at all it is possible for him / her to find it) that the rear wheels tend to move sideways more compared to forward movement.
