Why do those who I book to travel with me get the TSA Pre Check on their boarding pass when only I have the KTN? Even a stranger, a co-worker whom I had never met in person, got it. My wife and her friend got it when booking their own flights and traveling without me and neither of them have a KTN.
TSA can grant pre-check to anyone on any given flight, whether they're a member of the program or not, based on their own internal criteria. (There is currently a bill in Congress that would end this practice and restrict it to members only.)
The fact that they are traveling with a registered member may or may not have been a factor. As far as I know, TSA doesn't reveal publicly what factors are considered.