If you get SSSSed one way, you'll probably get it the other way, too. I think it's assigned to the whole itinerary, not to an individual passenger or instance of checkin.

I got my first SSSS in a couple years in January on a EWR-IAH CO flight. I got it going both ways, while my fiancee, who was originally on the same record (we split them to try to increase EUA chances), didn't get SSSSed either way. Perhaps splitting the reservations caused it, but I had split reservations on a similar EWR-IAH flight a couple months earlier without getting SSSSed.

I think the percentage of SSSSes that are random is growing. Now it seems that folks with Arabic-sounding names and/or truly bizarre travel habits are the only ones likely to get SSSSed on a regular basis. I have one friend whose last name is Ahmed who gets it almost every time, and another friend (HK-born Englishman) who usually has at least one RTW ticket going in each direction to bounce him around between SFO, NYC, BOS, LHR, MLA, ATH, HKG, MNL, PER, and SYD.

I think the old-school one-way/last-minute-purchase/no-ff# criteria are fading into the past. I haven't gotten SSSSed on any one-way or circle tickets I've bought in the last year, nor on the walk-up ticket I bought on UA a few weeks ago.