Ejections usually happen after the aircraft becomes unflyable. Birdstrike is a common cause, see this example: https://www.youtube.com/watch?v=H2zB7Z-b6Kc

The jet ingests a bird shortly after takeoff, causing engine damage and shutdown. Pilots try to restart the engine, after unsuccessful attempts they point the aircraft towards the nearest open field and eject. There wasn't much else to do. At low altitude, low speed, and without engine power and a place to land, the jet was going to crash one way or another.