I have heard that shock from the floor and some types of electrical interference can set them off. Also, I've heard screeners admit that kicking the machine will set it off. Back in the pre-9/11 days, I would set off metal detectors in places with dry air (like Denver) and wouldn't in more humid environments. ...just empirical data.

There is a metal detector in the office building where my gym is located. It is used for visitors going up to upper floors where there is a government office. Every morning, I walk between a baggage X-Ray machine and a metal detector (not through it) spaced about 5' apart. This time of year, I'm dressed completely in Spandex and synthetic wool in addition to my running shoes and running watch. Since the metal detector is on my right and my watch is on my left hand, the watch is a couple of feet away from the metal detector. Guess what? I set it off almost every day -- and I'm not even walking through it!

Next time you get a retaliatory secondary, look down as you walk through to see if the screener kicks the metal detector.