The cause is simply that your page has become popular on Stumbleupon. This usually means that people like it, so congrats. Basically the more toolbar users who visit your site and give it the thumbs up, the more traffic you will get.
I've found myself getting more and more SU traffic, and I believe this is because SU itself is growing in popularity. The traffic bursts you get from it tend to be more sustained than getting Dugg or Slashdotted, and they may even take weeks or months to tail off, depending on your content.