Or they’re in a hurry and didn’t realize the door was ajar when they left the vehicle.

Pretty normal for this to happen at the airport, especially when you’re carrying a bunch of bags, juggling kids, etc. Normally, its not a big deal, because the cabbie will pop out and close the door if they get the alert. But here, there’s no remote way to close the door and no way to signal the rider that it didn’t close completely.