Mostly I'm fairly terrible as Reaper, but occasionally I can do something that looks like I know what I'm doing, and this was one of those times. What you can't see is that short before this, I'd teleported up from behind a wall onto the girders overhead the second point, and walked over to that Junkrat from above before dropping down to take him out.
My mistake is that I still tend to back away. Never back away as Reaper. Ever.