This is conjecture on my behalf...

We know that there is a certain proportion of selectees on each flight. So if CAPPS doesn't flag enough people, the system begins to assign random selectee status to fill up the quota.

Does that make sense?

I'll never understand why I got SSSS'd flying from Columbus to Honolulu on a regularly purchased round-trip, purchased online a month before departure. I was a Silver Medallion frequent flyer on a route I've flown many times, using the same credit card I always use. I just figured that they had to fill the quota somehow.

Yes, I'm still bitter about that particular incident (they were wanding my bare skin, apparently determining the iron content of my blood.)