This is happening to me too. I think it might be closing pages that I'm not looking at in order to save memory, then loading them again when I view them.
Sometimes though, it reloads everything at once for no apparent reason.
I find this auto-reloading very obnoxious, it causes several different problems. Lost typing, losing the previous version of a page that has changed, and so forth. I would really like to find a way to disable it.