Can I reduce how Mobile Safari refreshes web pages when multi-tasking or browsing in multiple tabs?
What you're experiencing is the iPhone flushing pages from RAM to free up memory. There's no way to prevent this from happening, but some things that can alleviate it are keeping less pages open, and being mindful of the current number of apps that you're using that are taking up memory. As iOS needs memory for the current app, it'll kill off less recently used apps/pages.