Chemically, nothing, if by "moisture" you mean water, because water does not break the dynamite down chemically. Physically, because evaporation is a cooling process, the presence of moisture reduces the chances of accidental detonation. But moisture does cause the outer covering of most dynamite sticks to weaken, causing spillage (that is why many dynamite sticks are dipped in wax). For example, if dynamite is stored in a leaky barn or shed, through the years the entire building can become unstable; the dynamite leaks, spills, and then dries out. In rural areas, whole estates have been condemned because of careless dynamite storage.