I've done a little reading on this and it seems they are spawned when the Empire launches a 'manhunt' for you, ironic being a female character, but we'll skip over that.
TL;DR, When your wanted level is max, a manhunt starts, and the empire locks down the planet and deploys Death troopers. You can kill them to clear your wanted level or use another means like corrupt officer.
Ref: https://gamerant.com/star-wars-outlaws-where-find-death-troopers/
Once your wanted level maxes out, you'll get a notification that the manhunt has begun and your wanted level will set in at the maximum.
Now, you can only remove your wanted level by paying off an imperial officer, dying, or taking out the Death Troopers who have just arrived on the planet to find you.
During this time, the Death Trooper event will appear on-screen and your distance to the Death Troopers will be shown just underneath. Their exact location will also be marked on the map, making them easier to find.
During a manhunt, roads will be cut off with shields, and imperial patrols will be everywhere looking for you. Should you get to the Death Trooper's location, there will be three present with one officer
Once you defeat the Death Troopers and officer, use the key card the officer drops to get rid of your wanted level, end the manhunt, and get the They Live Up To The Name achievement.