Safari keeps refreshing itself
Safari on Lion exhibits similar behaviour to on IOS, in that it can decide to arbitrarily drop a loaded page in order to save memory, regardless of the fact that you likely have many many more times more Ram than on your iPhone...

When you switch back to a tab that has been dropped, it will then proceed to redownload it to get it back to how it was.  This is irritating, pointless, and potentially disruptive depending on exactly how the page was formed.

Sadly, before you ask your next question, no, there's no way to stop this: