Random? Nonsense.
On a recent late-night YUL-LGA leg, while waiting to clear security in YUL, I noticed the 2 pax in front of me both got SSSS (they hadn't noticed it on their BP) I noticed mine when the agent reprinted my BP after my upgrade, but then I also noticed the 2 pax behind me bot got SSSS as well. So 5 pax in one security line with the "random" SSSS? Bull. It must have been a man-made glitch to cause that many pax to be assigned extra screening, in a row, versus the so-called random selection process. What was funnier was the reaction of the security people who called out to one-another, rolling their eyes, "We've got another one!". I heard this a few times that night waiting in line - it would appear the security people think as little of the SSSS as the pax.