Secondary Security Screening Selection. Enjoy your pat-down. Happened to me once when I was returning to the US (in the European airport). I asked the person giving the pat-down (in a sort of closet - thankfully she didn't tell me to strip!) if I had done something, and she replied, in poor English, "It's for your safety." Yeah okay.

As long as you're cooperative and didn't give them an actual reason to do it, it shouldn't be anything more than an inconvenience. A crappy, anxiety-inducing, and arguably-unconstitutional inconvenience, but an inconvenience nonetheless.