WordPress causes it; it's standard behaviour on the first login, but shouldn't happen again.
I saw it for the first time when I registered my first test user, but then I setup another test user and it didn't happen.
If you have a user that is already created, it seems to happen if you logout on certain pages and then log back in, but it doesn't seem to be consistent. For example, if I click on the Image Categories menu, choose a category, choose an image, logout, and then log back in, it seemed to happen pretty consistently for me, but not all of the time. And now I can't reproduce it.
I was also noticing that the menu at the top right was changing when I went from page to page. On some pages it would show the Login/Register menu option, and then on other pages it would show the Logout menu option.
It seems that there might be a bug, but I can't seem to get it to act consistent enough to track it down.