I have had SSSS once. I extended my stay - I was supposed to fly home let's say Thursday night, but Thursday morning I changed my tickets so I would fly home Friday night. When I checked in I was specifically told by the checkin agent that the change was the reason for the SSSS - I was taking a flight I had booked the previous day. She, and everyone else involved, kept apologizing and explaining. I went through a separate security line, diverted there by the "look at your passport" person you go through first in US airports. This was actually a feature since I got whisked to the front. Then I stood in the middle of a big open space (other passengers kind of stared at me, which I didn't like) while someone searched my carry-on reasonably thoroughly - I've had more thorough seaches post x-ray - and confiscated my face lotion.
On subsequent trips I didn't get it again, lending credence to the "last minute change" explanation. BTW I have heard for folks who got it randomly, reprinting their boarding passes at a kiosk resulted in an SSSS-free boarding pass. I haven't tried that myself and I don't know that I would bother - it was probably a net benefit to have had the special treatment (though that face lotion had been through lots of screenings before and was never confiscated.)
Just had another SSSS and decided to update this answer. There were several changes this time around, some of which may have been about time passing, and some because this was in YYZ and the other in SEA:
- I was not able to check in online. When I arrived to check in at the desk, I asked why I couldn't check in online (I was worried that it was because of my upgrade) and she told me it was because of the SSSS. She also showed it to me on my boarding pass and said "be sure you get two stamps. Immigration here (the usual place) and security here"
- It did not get me to the front of the line and I was not made to stand somewhere special
- My bags went through the Xray but weren't searched. I went through the metal detector and then the nude o scope.
- the longest part of the whole process was the CATSA folks looking for the stamp
I think this one was triggered by flying home from LAS landing at 6am, and then turning right around to go YYZ-SEA less than 12 hours later on a different airline, when I live here. That is a YYZ-LAS-YYZ trip smucked right up against a YYZ-SEA-YYZ trip. Also this YYZ-SEA trip was bought on relatively short notice (a few weeks.)