I have gotten the SSSS before when I was rebooked due to a canceled flight. Friend on same itinerary did not get it. Friend on seperate PNR did get it.

After the second flight canceled, was rebooked on a third flight. This time I didn't get it, nor my friend on the same PNR - but the frind on a seperate PNR was selected for double punishment. Each time they initialed or otherwise marked the SSSS - which was eventually checked for on the one SSSS boarding pass which was actually handed in.

Similar experiences have occured on subsequent trips.

I couldn't determine any pattern. FF versus inFF. Credit card vs. award ticket. Gender (surprisingly, the Female members of the party got the SSSS more than the male). Checkin time. Checkin method. None really seem to make a difference as far as I can tell.