Randomly one-shotting survivors as Billy?

JesseJH28

I have no idea how this works. It happened twice this match to the same survivor, once on a gen and once coming out of a locker. I only used a basic attack each time. Unfortunately I only captured the one out of the locker as the chainsaw sound is what really threw me off with that. There was no sound from the chainsaw when I downed her at the gen, but she went down with one hit similar to how she did here:

https://youtu.be/kYUznSEY_kE

Not really sure what this is all about but I haven't seen it anywhere else on here, so I apologize if it's already been posted!