Issue is 'rare' but engineers are on the case
A bug affecting all versions of Borderlands 2 is resetting Badass Ranks right back down to zero, stripping the character of its bonuses, including unlockables such as customization skins. Even worse, the Badass challenges are still flagged as completed, meaning players can't try to re-earn them.
Fortunately, Gearbox has acknowledged the issue and, while it maintains the bug is rare, is currently working on a fix. It seems that, if you're affected by the glitch, all you can do right now is wait for Gearbox and hope your progress can be restored.
"Our engineers are looking into this, and the information you've provided has proven to be a great resource while we investigate," Gearbox stated on the forums. "At the moment, it seems to be a fairly rare issue and we'll keep you updated on when we resolve it."
Hopefully it gets sorted out quickly ... and that it doesn't get me. Now I'm paranoid.