Now this is not statistcally tested, it's just coming from the vibe I've been getting from all the mining I've done:

Mining nodes will become "nearly exhausted" after either a certain amount of gathering activities is made on them, or a certain amount of time lapses. Once it becomes "nearly exhausted", it will cause the deposit to collapse after just a few more gathering activities on the node (in the time-case, just once). This is independent for every node.