Why are summers especially rainy in southern and eastern Guizhou?

Summers in southern and eastern Guizhou are especially rainy because these regions are open to the influence of the moist maritime air mass during the summer months.1

This exposure leads to a distinct summer maximum in rainfall, which accounts for nearly half of the province's annual total. Most of this precipitation is generated by atmospheric frontal activity, though some also results from convection or condensation. Generally, the province experiences high relative humidity and frequent cloudy and rainy days throughout the year.1