I fly several times a year from ZRH to JFK, out of 20 or so trips, I had SSSS on my bp ONCE--it is (in general) RANDOM. They send you to a special cubical for further "checking"; they (at least AA has) a list of names of those selected which they check-off (it looked like there were 15 names).
Even though ZRH has NO TSA, my bps from ZRH to JFK have TSA PRE-CHECK on them.