The problem is water trapped by capillary action.

…within the slate

Capillary action also works at the microscopic level. It can be powerful and intrusive. Fed by a persistent water source, capillary action can drive water from a tree's lowest root to its highest leaf (fig. 5). And it can drive water through a stone's pores, commonly seen as rising damp in walls (fig. 6). The implications for slate are significant, particularly if the slate in question is characterized by high water absorption: Water trapped on one part of the slate can reach well into the rest of it. (This explains, in part, why some slates age pervasively.)