I assume you mean customs, not immigration. I don't see how immigration could hold anyone once they've determined that the person is a US citizen (unless they see an outstanding arrest warrant.)

I experienced the 2ndary x-ray bag scan once when coming back from the UK with three checked bags plus carry-on (mrs. ralfp was flying on a connecting flight in Y; I was non-stop in J). I assume the large quantity of luggage is what triggered the 2ndary.