Borderlands: The Handsome Collection Unlock Bundle is currently free in the Xbox Store, and no one seems to know why. First spotted in a Reddit post, it's not the first time that Microsoft has (presumably) mistakenly listed a game for free in the Store.
Back in June, Fallout 4 was accidentally made free. Microsoft ended up revoking the licenses from anyone that claimed it in the short period that it was available, but the company did offer up $10 credits to the Xbox Store.
The Handsome Collection includes two games: Borderlands: The Pre-Sequel and Borderlands 2. It's entirely likely that this was a mistake and licenses will be revoked, but users don't have anything to lose by claiming it while it's free. The best-case scenario is that you get a free game, while the worst-case is that you don't.
Update: Larry Hryb (Major Nelson) has confirmed that Microsoft is inviting all users to play online with friends this weekend, so it is offering Borderlands: The Handsome Collection until October 30.