Why did enemies spawn around me when I fast traveled after the latest patch?

So it's happened a few times, but never anything major, like an out of place mage or something.

Well, this time, after the latest patch (from 1.4.21 I believe to 1.4.27), my lady necromancer fast-traveled to Riverwood and was greeted with:

"A challenger is near!" "I'll mount your head on my wall!" "You never should have come here!" "BAR!" "QAPLA'!" (or whatever it is draugr say)

Said the 5 Bandit Outlaws, 4 Bandits, 3 Draugr Scourges, Fire Mage, Dremora Kynreeve, Restless Draugr, Wounded Frostbite Spider, Thalmor Soldier and Draugr Wight. It was a tragic day for Riverwood, except children who are obviously invincible. After I heard a dragon overhead while being chased by three frost atronaches I reloaded.

So this time I traveled to Embershard Mine, which wound up more manageable:

wat

I traveled again to be greeted by about 9 assorted draugr, then again for two more. Then the dead stopped following me.

I should also note that these aren't random spawns. One of the bandits was carrying the Note to Rodulf and the dremora had the Key to Mehrunes' Shrine. When I went to White River Watch that bandit wasn't there.

My theory
This is also my first necromancer. What I think happened is that the latest patch cleaned up and resurrected some bodies, but some of those were tagged to be following me (perhaps getting stuck in an area summons can't leave?). I have a habit of "renewing" my draugr summons every time I come across a decent candidate because my character also focuses on lightning and illusion, neither of which helps much with undead, so that would explain the dominance of draugr. But I have a sample size of 1 here.

Is that accurate? Has this happened to anyone else, and is there are a way to avoid a battle royale in the future?

One of my mage characters specializes in Conjuration - Necromancy too and this happened to that character upon updating to version 1.5.26.0.5.. It didn't happen to my other non-mage character that doesn't use Conjuration - Necromancy spells.

With my Mage-Necromancer character, one named NPC, Sebastian Lort, was among the enemies that ambushed me upon fast travel. I remember killing him in his related quest, and then reanimating him as a Dead Thrall (permanent zombie), and that he, as a Dead Thrall, later died.

I do remember many of the enemies that spawned upon fast travel, as being NPCs that I have previously reanimated. I guess your theory is correct that the game may have resurrected previously reanimated enemies upon updating Skyrim.

This issue appears to be self-fixing too. Once I have killed all enemies that spawned upon fast-travel and re-reanimated Sebastian Lort =), no other enemies ambushed me upon the next fast travel.

Edit: Sebastian Lort, who died again after I re-reanimated him, ambushed me again around 5 fast travels later. I decided to eliminate him this time and not reanimate again and see how it goes.

Update:

The v1.9 patch has a fix for this:

Fixed rare issue with NPCs and creatures respawning improperly after player fast travels

I had the same thing happen to me actually. Was very odd and just happened with the newest patch out today for the PS3. I counted up and took inventory of the dead bodies the first time it happened (reloaded and tried again and again with slightly different results), from Riften to:

Whiterun (first trip): 3 bandits, 1 bandit thug, 1 fire mage, 1 ice mage, 2 spriggan matrons and a cave bear

Random keep (second trip): 3 bandits, 1 bandit thug, 1 fire mage adept, 1 ice mage, 2 spriggan matrons, a cave bear, a wounded frostbite spider, 2 dragur, a snow bear and Hajvarr Iron-Hand

Fort Haggstad (third trip): 3 bandits, 1 bandit thug, 1 ice mage adept, 1 fire mage adept, 1 wounded frostbite spider, 1 cave bear, 2 spriggan matrons, a dragur and Hajvarr Iron-Hand (along with the Imperial Soldiers and a dragon which are part of the area, my horse and Ilyia....quite the free for all that went on).

All the times I noticed Hajvarr Iron-Hand, he just stands there, nobody attacks him, he doesn't attack anyone nor say anything. The fire mage is usually one of the last things to die and causes a lot of fire explosions. Ilyia died the second time, Frost my horse died the third time... Odd...