Yuck. Been there.
When the nice TSA person checks your ID and marks the boarding pass, does the mark look something like "S" or "SSSS"? That means that you are very special today!
OK, it actually means that you are subject to additional screening, in the TSA area and possibly again at the gate. (Fun times. "Please step aside." Then a TSA person shows up on one of those airport cards, lifts out and sets up a folding table, and does the whole TSA inspection drill by the gate, right down to "belts and shoes off, please.")
They appear to run an algorithm that looks at peoples background and if it finds an association (same employer; lives next door... who knows?) with someone on a watchlist, TSA records for you are flagged. They did stop after several years, fortunately.
Spent Saturday night at my friend's house. His DW is a TSA supervisor. I specifically asked her about random selection. She said it is exactly that. Random. She admits that she doesn't know all of the triggers-similar name on no-fly list, etc... Obviously, you get further screening if the detector goes off or something suspicious in your bag. She did mention a certain former NFL QB who wasn't an A hole prior to his new found celebrity as a divisive former NFL QB, but now magically is. She also said Peyton Manning is super nice and friendly. The bolded above doesn't happen. The requirement to show your boarding pass going thru TSA check point went away some time ago.