It is thought that "deep" moonquakes are caused by tidal forces exerted on the Moon by the Earth and Sun. Some other sorts of moonquakes are thought to be caused by impacts, or by thermal expansion (when part of the moon rotates into sunlight).
However, there is another type of moonquake that occurs just 20-30 kilometers below the surface. Do we know why these quakes occur?
The article I linked above suggests that the cause is currently unknown, but that article is a few years old and is just a popular news article. As I am not familiar with the literature on this topic, I would like to know if there are any well-substantiated hypotheses about the cause of shallow moonquakes that have been put forth.