A [relatively] sharp-eyed US immigration official in SJU a couple of years ago noticed that I had visas/stamps for some Arab countries (Egypt, UAE and Morocco mainly) in my passport. I was taken off into a windowless room and interrogated about why I went to those places, but was allowed to go once I'd satisfied the official that it is actually possible to visit many Arab countries on vacation and that there are lots of nice people and interesting things to see and do there
Anyway, ever since then I've been getting SSSS almost every time I take domestic flights in the US. It may be just a coincidence, but it doesn't seem like it. For some reason it never happens when I'm boarding international flights from the US.
On the whole, I don't really mind the extra inspection. In fact, sometimes it seems like a privilege. I connected through BWI recently and was delighted to be pulled out of the main line (which was 100s of yds long - the longest I've ever seen anywhere except in a couple of developing countries). Although the process took longer I'm pretty sure I made it past security quicker than if I'd stayed in the main line. And having to stand in some sort of orgasmatron and have puffs of air squirted at me was almost amusing.