They aren't too uncommon. I think they are formed when a some body that has broken up somewhere along the way impacts the moon. Think about Shoemaker Levy on a smaller scale. The moon went through a period of terrifically frequent impacts. Much has been covered up by the lava flows that form the moon's seas.
Possibly Trained, however the ones I've had pointed out always seem to be identical in size and in shape. Anything hitting the moon is unlikely to cause so many identical craters - even if some objects do split up on the way. Any such objects would have to hit straight on, otherwise the craters would not be circular, also objects splitting in two would have to split into two even-sized pieces to make two craters of the same size surely?
These craters do not show anything paranormal or even artificial maybe but they are surely interesting and as far as I've read in the literature they are still quite a mystery?