This has been happening to me too.
I kinda figured it out, the bitcoin client automatically makes "New..." active focus, thus when you press any key with the bitcoin client in foreground, it makes a new key. Has happened to me 6 times so far.
Edit: I also noticed it does make a new address when the application is first started and or receiving a transaction (I got bunch today and there were several new addresses).

This was on a newly upgraded installation with the latest bitcoin available from bitcoin.org (win). The AppData folder was the only one remaining from previous installation for wallet.