My understanding is that insufficient tension on the spokes is the main cause of a wheel losing its true. The spokes lose tension every revolution as the load shifts on/off of them. If they get loose enough, they can unscrew themselves leading to a loss of true. A poorly made wheel can also lose true very quickly if the spokes haven't been 'stress relieved'. You'll hear a popping and pinging sound as you ride on the wheel for the first time as the spokes twist around. On the other hand, too much spoke tension can damage the rim or cause the spokes to pull through.