Is this happening to you on domestic or international flights?

This has happened to me several times, all on international flights. There are two reasons it happens to me:

1. If it's been more than a few times since the last time I flew international, the system wants me to check in again at the airport so my passport will be swiped. It's important to swipe it in the kiosk as opposed to having an agent enter it.

2. The country of my destination requires a visa.

It happens to someone else I know a lot too. When she gets her BP at the airport, it will often say SSSS on it, indicating that she's subject to secondary screening and that's why she cannot check in online.