As a non-US citizen who travels very frequently to the US, I have ocassionally fallen foul of the SSSS, but there seems to be no real pattern to it.

I typically get selectred when I'm travelling on a single itinerary AND I don't have my AAdvantage EXP number in the booking (either because I'm not travelling on AA or because I'm trying to top up my BA miles balance). But there are plenty of occasions where both of these factors are present and I'm NOT selected for SSSS.

That said, I've really got no complaints about the process. In many cases getting selected means that I get to the front of the security line anyway. And the process itself isn't exactly time-consuming.