So from the word go, I have been using necromancy skills, and continue to do so. Recently however it seems my previously raised people and creatures are coming back with a vengeance.

About 50% of the time when I fast travel, I encounter one of my past minions, who are now hostile, be they a Falmer Gloomlurker in Whiterun, a Drauger outside Ancestor's Glade, or my latest one which was the Chaurus Reaper outside Lakeview Manor.

What's even more peculiar is that once they have been defeated, no level of raise dead spell will touch them.

This is my first necromancer play through, is this a known bug?

It may be related but on my old computer I had a glitch where mammoths sometimes with giants would occasionally spawn when I entered a cell. Even Interior cells which looked weird. You could sometimes see them fade into existence then they would follow me. Occasionally they would be netches and later with a new game gargoyls. I never did figure out how to fix it. It could happen even with a new game during the wagon ride.

That is strange, indeed! I've had defeated dragons (dragon skeletons) follow me about when fast traveling, but no experience with necromancy. Does the http://www.uesp.net/wiki/Skyrim:Skyrim have any info on that? They're fairly thorough