Props can and do come off in flight. That's why DJI added prop locks to the Inspire props and changed to the composite hubs on P3 props. DJI claimed that active braking could make a prop come off. The other possibility is a glitch in the ESC. If one of the three motor wires is disconnected the motor will lock up and the prop will fly off unless it was really tight.
Here is a video of trying to make a prop fly off by manually stopping the motor. It was difficult but possible. At the end, you can see what happens if you disconnect a motor wire.