Rising humidity is caused by the ability of water to penetrate the walls and rise from the ground upwards through the capillaries present in the materials constituting them. The water goes up the capillaries, until the forces at play find balance, and is then disposed of through the normal process of evaporation through the surface of the masonry.
This triggers a process of degradation of the elements that make up it, causing widespread presence of spots and saline efflorescences and detachments of plasters from the masonry.