Safari keeps refreshing itself

Safari is keep refreshing itself multiple times a day which is quite annoying. It prompts a message to me: "Force Reload" and then it reloads every tab in every window. This is unacceptable.

Any idea what is going wrong?

(Safari 5.1.3 / OS X Lion 10.7.3)

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: