Database breach?

Had to change my login after I was found that my password was taken in a database breach. Can someone fill me in on what happened there?

When you set a password on Derpibooru, the site generates a hash* of that password and checks it against a list of hashes for passwords that are either weak or have been released in database breaches for other sites. This is a security measure intended to prevent people from reusing passwords that have been compromised on other sites. You can read about it here
 
*Please note that the hash generated for this check is used exclusively for this check, and is not stored anywhere. We use a different, significantly stronger hashing algorithm, along with per-user salting, to store passwords in our database.

I knew you guys used HIBP. Thanks for helping to protect my security.