Just to add, it's not limited to US citizens. Both my wife and I have had it, on separate trips (we pretend not to know each other when one of us has it, just in case the other is drawn in).

I find it happens to me on about 1 in 20 trips. I presume it's triggered by the US Government as part of the APIS information check. Strangely, I normally get an X on my entry form arriving in the US, but the last time I got SSSS I didn't get the X and cleared much more quickly. Personally, I'd far rather get SSSS and no X than the normal way around.