It's a memory issue. Web pages take an enormous amount of memory to render, so when you switch to another app and it needs memory, webOS will purge the browser memory to make space for the new app.
I bet the Pre+ doesn't see this as much as the original Pre because of the extra memory.
You are 100% right about this stupid feature being a Memory Issue. Ever since you mention this, I have been watching my browser more closely and the Auto-Refresh happens when a lot of Web-Pages And/Or Apps are running. Even though I understand why this keeps happening; it doesn't make it any less annoying. I really wish that Patch worked for me!