I think this applies to all US domestic flights. There is a computer system that supposedly uses several different criteria to mark people out for special screening. You can sometimes tell that it's going to happen to you because you get the large letters SSSS printed all over your boarding pass. So if you meet the criteria, you might well get selected several times in a trip.
Your story about the knife just makes me laugh. You have to remember that much of this is done simply to make us feel safer.