Some weird things are happening on Steam right now, with the entry for Borderlands DLC: Claptrap's New Robot Revolution showing randomly on the last played list. I did a little research inside the Steam databases and here is what I've found:
The Borderlands Steam Database Entry shows a recent change in the DLC list, with the new addition being a duplicate of the 4th existing DLC. The duplicate's Entry shows that it underwent various localized tests, and is being turned into a DLC class (previously Game), with Borderlands being its hierarchical parent.
Also, the Entry for something called "Gearbox Comp" lists the recent addition of our mysterious duplicate AND another, unrevealed thing.
From the programming point of view, it may be a placeholder for a new DLC for any game (so including BL2), as it should take no more than a few number swaps to attach it to another title. A decoy then? Or maybe a simple clean-up of a relatively old game?