What exactly "earns" someone the SSSS? It looks like common last names, one-ways, overnights, right?

I ask because I got my first one (that I remember) early this year. I was told that I was "randomly selected by the computer", but the funny thing was that the UA system was down and my boarding pass had to be hand-written. The fact that it was hand-written probably did it, but I find it odd that the agent still tried to blame the computer.