If they are assassins, then they should have a note on them with their orders, explaining who sent them and vaguely why they want you dead. Just playing through I think you will pick up people who want to kill you. I was surprised this morning by a Thalmor hit squad outside a bandit hideout. Up until then on that play through, I haven't even seen any Thalmor before this. (okay, one dead guy. but he was like that when I got there, I swear!)
I'm not very far in the main quest either. No spoilers, but I had just got back from Kynesgrove and I wanted to clean up my quest log a big before continuing on the main quest.
Nah, there doesn't seem to be any apparent clue on who it is. All of my notes say the same generic thing:
As instructed, you are to eliminate [Character's Name] by any means necessary. The black sacrament has been performed - somebody wants this poor fool dead.
We've already recieved payment for the contract. Failure is not an option.
-Astrid
It's a nice touch, but like I said, it seems to happen so much that I thought it might be a glitch. It's been almost once every in-game day lately.
EDIT: I just looked through some of my stuff and was reminded that I got a "Letter from a friend" talking about how I caused a stir in Ivarstead by using a shout. Maybe that explains why I've been attacked so much on my way there?