It can be a random thing. after flying over 250 segments on AA since SSSS was initiated, I got a SSSS BP. And judging from my previous random SSSS from AS, I surmise that the SSSS applies to both ends of a round trip. This is patently stupid, as is the printing of SSSS on the BP anyway. I did extract a small sense of personal revenge (we get off on the small things) by pointing out that SSSS implies that normal security is not adequate. And apparently I was polite enough about it that I did not get too much extra retaliation.

The airports involved were SJC and BOS. The procedures were quite different, with SJC taking forever, and conducted by the village idiot.

As others have stated, the other reasons for SSSS are airline rules such as one-way tickets (seems to be less than it used to be), and the most idiotic of all - re-booking for a cancellation.