What causes "shallow" moonquakes?
According to the paper The lunar moho and the internal structure of the Moon: A geophysical perspective (Khan et al. 2013), suggest 2 causes for the less frequent, yet more energetic shallow moonquakes:


Similar tectonic movements to intraplate quakes on Earth, also mentioned in the NASA paper Interior of the Moon  (Weber, 2013), but this seems unlikely due to the lack of plate tectonics on the Moon, but in an earlier paper, cited by Khan et al. (2013), Shallow moonquakes - How they compare with earthquakes (Nakamura, 1980), suggests that slippage in near surface fracture zones are a possible mechanism.
An alternative explanation suggested is the



  possibility that shallow moonquakes may
  instead be triggered by objects emanating from outside the solar system,
  such as nuggets of strange quark matter.


However, as there have only been 28 recorded since we've been to the moon, it is hard to exactly say.