How can the tyre slips on rough roads.
Although I can only comment from my personal experience, I thought a picture might help. The picture at the bottom of this reply is an intersection near my apartment. As you can see the pavement surface is smooth on one side and rather rough on the other. In some parts of the rough patch, the pavement and potholes are down to bare earth. As such, when braking, depending on which tire is on the rough patch, the anti-lock will engage because the AWD detects well spin. When the anti-lock engages, it sounds like the tires or brakes are grinding.
Of course this is just from my actual experience. Not certain if the roads are better or worse when the condition happens for you.