I just started a new character on 11th April 2012. After completing the kill-a-dragon-at-western-tower quest, and buying Breezehome, I now appear to have a greeting committee of bandits after fast travel.
First, upon leaving Breezehome, the bandits from Embershard Mine appeared and were helpfully cut down by guards and citizens. This was the first go-kill-guys-over-there quest I picked up in the game, the bounty from Falkreath, and the quest was completed as soon as all the bandits died in Whiterun. I fast travelled to Falkreath to hand in the bounty to Nenya (2 transitions - Whiterun to Falkreath, Falkreath to Jarl's house). I then fast travelled to Riverwood, then to Whiterun (total 4 transitions), when the bandits from Valtheim Keep spawned and were killed by guards (Companions quest; incidentally the second go-kill-them-over-there quest I picked up).
This may be a patch 1.5.26 bug, related to the issue described here: Why did enemies spawn around me when I fast traveled after the latest patch?
I'm using Windows 7, Skyrim 1.5.26, SkyUI, and a couple of other mods (alchemy storage, weapon crafting, better horses).
Is anyone else getting this or similar behaviour?
Added Observations: It seems that going into Breezehome for the first time after getting the quest to kill the bandits from Halted Stream Camp from The Bannered Mare spawns the bandits in the house. After some testing, it appears that fast travelling to Riverwood will cause the quest marker to appear in Whiterun, and the bandits to spawn there after fast travelling to Whiterun. I also notice that the spawn point seems to be the doorway of Breezehome. What in Tamriel is going on?