Why is this happening? Is it just a random encounter? They had a note on them but it didn't say who wanted me dead.

Random encounter. Happens to me very often- I have like 15 of those notes that I've collected from dead assassins, haha.

It's a random encounter that might (or might not) be set off by stealing something (seen or unseen).

They came after me because I said something wrong to someone, believe it or not.

They svck.

Random until you join brotherhood.

The hired thugs on the other hand... they were actually sent by an NPC you wronged, and you can locate who. I just wish there was a dialogue option to confront them about it besides stabbing them and hearing them scream.

No, just a random encounter that you ...randomly encounter until you join or destroy them.

We're gonna teach u a lesson:)

These guys came to a house that i built right up onto the deck... It was awesome:) I'l post a pic when I can.

http://imageshack.us/photo/my-images/19/tesv20120422131836327.jpg/
http://skyrim.nexusmods.com/downloads/file.php?id=13862

It's the same as Hired Thugs. People hire them if they have a problem with you.

For example, I read a note after some Thugs came after me and it turned out that a Guard had sent them because I'd stolen something as part of a quest!

I can't go anywhere without being attacked by an assassin or thief. I do like seeing my Nord's name on the notes though.

No matter how many assassins my Nord effortlessly kills, they keep sending more. I'm waiting for the DB to run out of assassins any day now...

They'll probably be sending me soon.

I better start training more!

Sweet Mother, Sweet Mother, send your Child unto-oh! Hey! I was just talking about you and how this totally isn't what it looks like.