While Scrivener is open, writing.history is constantly (once per second) being touched, updating the modified date and causing dropbox to perpetually sync. Restarting Scriv stopped it. I’m not sure what caused it to happen, but the debug window showed “checking for documents to unload” repeatedly.
What is your setting for “Save after period of inactivity”? I think it defaults to 2 seconds, which can be too fast for Dropbox. I changed mine (based on other’s recommendations) to 10 seconds. That seems to keep the constant syncs down to a minimum.
I should have clarified this was while I wasn’t using Scrivener at all. It was open, but I wasn’t writing in it or altering any files.
Oh. Nevermind. I got nothing for you. DOH! 
This is why I recommend turning off Dropbox sync whenever you have Scrivener open. Once Scrivener exits, turn sync back on.
Other idea: Set backup on exit, and sync with your backup folder, not your project directory.