Alot of the time you see it when players shoot too hard.
Doesn't matter what the cloth is although it's more common on a non-worsted cloth like Championship, Mali, or Forstmann's.
Basically, the ball bounces from the back of the pocket, hits around the edge and stresses the cloth. Without something there to dull the effects like taking the edge down, or adding a backing to it to help deaden the blows it's like hitting your finger with a small hammer......eventually something has to give. And it won't be the leather pockets, the slate or the balls...
Not every installer takes the extra time to add something into there, or file down sharper edges or tell you about the effects of not doing anything. Most poolhalls seem to have some filing done to reduce the problem among other things.
Personally, I don't feel a table is finished until there has been some extra work done around the pockets, but that's just me.
You don't generally see it as much with rubber or plastic pockets, because they give before the force of the shot, so they absorb some of the momentum before dropping the ball down. Leather pockets have an iron inside that doesn't give.
Poolhalls also generally have someone that knows what they are doing, so they doctor things up while they are already doing the job, so that they don't have to deal with it down the road. Alot of the guys working on home tables don't know the difference.