White American here. In the past 3-4 years (probably a couple dozen flights) I have gotten some level of patdowns and hand-swabs literally every single time I go through one of those millimeter-wave machines. Every time, without fail.

My belief is that it's because I had gone through some pretty substantial weight loss in the past, and so my body skin/muscle/fat density is all screwy around the midsection, and thus it sets the machine off.

My white buddy got stopped and frisked traveling around Europe with me every time. He has a legit beard. They even asked if they could put their hands inside his pants (inside the waist band). Beards are suspect these days no matter what your background.

Discretionary secondary screenings can have a lot to do with appearing nonthreatening. If the guard has a quota to hit (even an unstated one) and wants to minimize effort spent on dumb secondary screenings, it pays to choose people who won't put up a struggle.

perhaps they profiled too many brown people and had to make their numbers look 'random' again (:

I've also been subject to secondary screening a lot more often than the GP, and am a white, native-born US citizen with a germanic surname.