I fly weekly for work and have not had any secondary screenings as of yet (knock on wood). My partner flies with me only for leisure trips and the past two times we've flown together (January and this past week), he's been flagged for a secondary security screening. Both times it was on the return flight to the US, one from Canada and one from Mexico. How are these screenings determined? Is he flagged in some database somewhere? It doesn't seem to happen on US domestic flights.

Any information would be greatly appreciated. Thanks!