If some program requests to start X11 and X11 does not work you can end up in the situation that X11 keeps crashing even though you press “Quit”. This is due to X11 (and friends) are launched from “lunchctl”. To stop this use:
launchctl list
This gives you a set of jobs under “launchctl’s” command. You want to remove the “org.x.startx” job. This is done with:
launchctl remove org.x.startx
X11 should now stop keep crashing… 🙂