There could be any number of reasons for why you're sent to secondary. Could be you're the lucky 10th person with a blue shirt. The main thing is to stay relatively cool, be consistent, don't lie, and don't be a smart ass. Most of their questions are rhetorical. They know your criminal history (to a point), know where you live, how many bags you checked and what color they are etc.. Land travel is a little different, especially at the smaller ports. I love crossing at a few of them just to BS with the regular officers.