Some possibilities.

1. There was an event that you didnt notice and the draugr spawned from it.

2. You are in an area where they can potentially spawn but you didnt have enough workbench or similar coverage to prevent spawns happening.


Ways to lessen this risk/issue.

1. Have multiple maps(seeds) specific for specific boss/tier fighting. This means you only have the initial neck/boar spawn event.

2. Building better protective pins with significant distance buffers on all sides(including ceiling. This way chances of clippers getting to tames is reduced significantly.

3. Better, wider radius inundation of workbenches or the so far around your base and in it that spawns are cut to nothing(or close to nothing.

4. Building your main base on a small island so far out from any other land mass that event spawns or other similar things never can occur due to being away from it all. so to speak. Make sure per point 3 that entire tiny tiny island is well bench'd.

5. Use mods that also help better protect you(dont really need if doing above though but its a choice at least).