Draugr spawn from Body Piles, much like Skeletons spawn from "Evil Bone Pile".

You can tell a Draugr spawner is nearby when you can hear large amounts of flies. In the open Swamp, Body Piles are your number one kill priority. If you can spot one far enough away, you can snipe it with arrows before it starts spawning Draugr. Inside of Sunken Crypts, you'll have to do this either in melee range, or with a Bow if you're peeking around the corner of a Muddy Scrap Pile (what you mine for Iron).

They're so dangerous because they're fully capable of spawning starred Draugr, both melee and Archers, but also Elite Draugr.. who can hit very, very hard if you find one that's starred.

Edit: as a reference, I'm using an upgraded Troll set (one upgrade from being maximum since I don't have a L5 Workbench yet), and a starred Draugr can hit me for anywhere between 14 and 85 or so damage. The higher end coming from a Draugr Elite.

If you find yourself running from a starred Draugr Archer in the open Swamp, do NOT run away from it in a straight line. Try to hide behind things as you run or they will get you.. and they hit in the 20s and 30s.