Swap Used Explained?

I have recently been watching my swap slowly filling up, over the period of 1 week, it gets close to 100%. Can anyone explain why this would happen? I thought the behavior of the swap area was that it is used, and then emptied on it's own? This is like a big glass glass of water, it slowly fills, I empty it, then it slowly fills again. Any thoughts?

* I should note, I'm not looking for a reason for the 100% usage as much as an explanation. As stated, I thought the swap area "self managed" itself. Increasing and decreasing as needed.