Why did a branch in a fork show up in the original repo?

Hi all,

Somebody created a fork of my repo (egil/bUnit) and then created a branch in their fork. Now I see the branch show up in my repo, and I wonder how that happened. The branch in question is https://github.com/egil/bUnit/tree/Packt-edits.

Did I do something myself? Or do I have some security settings that are too loose?

Thanks, Egil