More likely you stole someone's sweetroll.

I've had a couple of these, not sure that any light is ever shed on the matter. It's quite cool though when it first happens.

I was stoned and got really excited.

It's triggered by you stealing or murdering.

One time, Astrid was hired to kill me. I guess being dragonborn attract the wrong attention.

Well...anyway, i'm so very disappointed with the DB in this game. This is my first TES, and they seem just your usual hire to kill thug, except a little more fancy and professional. I thought they were some kind of chaotic lawful organization, getting rid of the dirty people in the world in the worst way possible and making examples of them. But nooooooo, let's kill the helpless beggar or the nice orc who can't sing...

They usually come if you've offed someone.

Not sure about the DB assassin but, if you get the "visit" of three heavily armed thugs, it will be because you robbed someone something. Skyrim people don't rely to the authorities if you steal something from them, they do justice by themselves.

My suspicion is it starts randomly after a certain dungeon is cleared but I'm not sure which one, I suspect it'd be a bandit dungeon with a named boss tho.


@ladyonthemoon

I think you can get armed thugs for other reasons, like quests where you can give an item to one or two certain people or completing quests in certain ways that upset some people. I think one would be the quest where Sven and Feadal(sp) both write up fake letters for Camilla.

I've had thugs being hired to teach me a lesson but not kill me... And the dark brotherhood is there to kill you though, my guess is
Steal = thugs
Murder = dark brotherhood

Honestly, you thought wrong. The Dark Brotherhood are and have always been hired killers. In Oblivion, they added a quasi-religious aspect to it (or rather, emphasized it much more than previous games).

Sometimes when you steal something you're hidden enough to not have the guards called, but not hidden enough to escape the vendor's notice. It's sort of like halfway failing a sneak check. When this happens they might put out a contract on you. For you it was the DB, for me it's usually a trio of steel-clad thugs.

I think this pretty much happens anyway when you go through the main quests whether you're good or bad...

Someone always wants you killed

I have had DB, Thalmor, and the thugs after me. Thugs were because 'someone' knew about something I had 'borrowed' from their house. Thalmor was because I am Dragonborn and a threat. The DB I am still attempting to determine why....perhaps because I have slaughtered all the Thalmor hit squads? I don't think I have randomly murdered anyone....in fact I know I haven't. If I had, the DB would be coming to recruit me.....

Thalmore Embassy - Diplomatic Immunity - Main Quest

That one sends 2 DB agents after you - BUT!

The last character I had was an explorer/scout (Lewis and Clarke idea), was NOT doing the main quest, and she had 'mapped' around the embassy and was on her way to the next location to 'map' when DB attacked and gave her a note.

SO ... I guess that DB agent hangs out near the Thalmore Embassy.

I've only murdered one person for no reason.... well I say no reason, it's that redguard farmer who lives in Whiterun who is full of himself and tries to make himself better than he is. He was sleeping in The Drunken Huntsman beside his wife. I couldn't get him alone because the shop owner followed me so I lost patience and killed the farmer....with a hammer. His wife sleeping beside him had awoken at this point (if I remember correctly). I got arrested and went to visit his wife afterwards. She never mentioned it.

Rude to me, loose your life....

It's just one of many random encounters.
A contract is a different matter completely. Contracts are very specific if you've stolen or murdered.
See here: http://www.uesp.net/wiki/Skyrim:Contract

The DB assassin encounter is not one of these two contracts. It's a random encounter.
I have one character who has not murdered nor stolen...assassin encountered.
I have another who has not done a single quest after leaving Helgen...assassin encountered.
I have one particularly shifty little git...contracts put on his head.

It's a random encounter, nothing more.
Use your imagination to role-play the cause. There is no wrong answer as there is no trigger for that particular encounter.
It's random.

First time it happened to me, i was pissed because I wasn't expecting it and had to burn too many pots to stay alive. I politely looked at who hired them on the note and went right over there and killed that person.

I stole a cabbage or something from his little garden. Eye for an eye and all that.

DB assassin randomly appear, you don't have to have killed anyone or stolen from them or have done a certain quest to trigger it, just some random dude wakes up one morning in hammerfell and decided to put a hit on you though a evil ritual even though they never met you. You'll never meet him or get to repay him.
Same goes for Thalmor hit squads even if they have a letter claiming you've been a thorn in thier sides.

It is pointless to ask such questions until script decompiler is available.

It's now sure that the DB wasn't sent after us by someone to whom you robbed something. My character has stolen nothing yet and he got the visit of an assassin:

- http://cloud.steampowered.com/ugc/651000053494825077/86FC7CB18F20364E929A0C9A728453A15CAC2771/

- http://cloud.steampowered.com/ugc/651000053494822259/0F39630C4463A2B540A66FF38485E8714AA6060C/

Shortly after I did a Thieves Guild Quest, I started getting attacked by assassins. I had a hunch who this was, did some investigating and found evidence in this person's home behind a locked door in Riften.