You earned it from this question: derby.log (etc) control in a webapp. You posted a 50, the target earned 100, this was under the old system, so that's a manual award rather than an auto-award.
For this kind of scenario you can always search your Reputation Graph, or if you want your own stuff just check your reputation audit. See every post where you lost reputation to a bounty, and check that you manually awarded it. This is actually one of the easier badges to figure out if you're wondering where you yourself earned it.
Or, as waffles points out in a comment... they already have badge origin support...