Why are assassins coming after me?

I recently started a new game as a mage type character. As far as I can tell I haven't done anything wrong yet, haven't stolen anything, etc. In fact, the only quests I've done so far were the initial main quest up to the point where I was appointed thane of Whiterun. Yet as I was walking through the wilderness to Ivarstead, I ran into an assassin who had this Dark Brotherhood assassin's note on him:

As instructed, you are to eliminate [playername] by any means necessary. The Black Sacrament has been performed - somebody wants this poor fool dead. We've already received payment for the contract. Failure is not an option. - Astrid

Who hired the assassin and why?

Solution 1:

It's a random encounter for you to get to know there is an Assassins Guild in the game. It happens always early in the game (so it's somewhat scripted) and later you get the option to join them or destroy them.

The player never gets to know who wanted you dead and you can't ask any of the Dark Brotherhood about it. It's not really that important.

Assassins will keep being sent by Astrid until you join the Dark Brotherhood, so if you want to stop getting assassins hired, join the Dark Brotherhood.

Solution 2:

It sounds like it is just a randomly generated encounter. There are many potential encounters that are generated while you are walking around outside, not all of them are triggered by previous actions.

Solution 3:

It's a randomly generated event, it does have something to do with the Dark Brotherhood. However, to join the Dark Brotherhood you must help Aventus Arentino in Windhelm complete The Black Sacrament by killing Grelod the Kind in Riften. The name of this quest is "Innocence Lost."