I am a level 12 and I just ran into this problem. I'm almost positive these thugs showed up because I stole items from Pinewatch (a bandit hideout), because after the first time they attacked the next morning when I was trying to sell the items in Falkreath (and beat my ass down badly), I fast traveled to another city to sell the items that I stole from the bandits, and after I did, I was not attacked in Falkreath the next day when I returned. However, when I returned to Pinewatch to steal the remainder of the items I had left there, the thugs were waiting for me when I walked out of the door of the cabin. This time I was more prepared. I had already gone back to a previous destination to retrieve my patiently awaiting housecarl , Lydia (who happens to be a bad *****), and I also thought to summon a flame atronoch. Having those two to help made the fight much more fair, and I also took the advice of some of you here and ran away and used some long-range attacks (fire-bolt) then finished of the one remaining with a sword. I would not say they are easy to kill but it's definitely possible. It might teach a low-level player a lesson about thieving from thieves.