In short, as boo1 said, moisture moves TOWARD dry and/or cool conditions.
The mechanics are a bit more complex than that for a building. For example, moisture generally first moves into the exterior finish materials by capillarity, or directly through a leak. It moves laterally and downward.
When a void or cavity is encountered, the relative humidity in the void or cavity is increased by the moisture intrusion. Now we get condensation. There is nothing to drive the moisture backward, so it is cumulative and progressively worsens unless the source is stopped.
The accumulated moisture then "attacks" the next dry area or cooler area. The same mechanics prevail and it will move to some other location, inward and downward. In the process, given reasonable conditions, mold will grow!