These are five of the six primary grounds for me seeing this kind of thing:

1. International travel document check needed to make sure the passenger has the required documents to be admissible to the destination.

2. Flight/booking changes/disruptions have a ticket being out of alignment with flights on the booking.

3. HaraSSSSment screening target.

4. Payment form/bank card verification check.

5. Ticket number missing or invalidated/blocked.