Why do you think this is Steam Client issue?
Boot in "single" mode and look in logs for clues, why Xorg crashes.
Please close, got the wrong project, sorry for the noise.
FWIW, it was a Steam Client Bug, see https://github.com/ValveSoftware/steam-for-linux/issues/4076
As of 21. October 2015 X is no longer starting on my machine.
I can see Grub starting, a couple of console log messages, a resolution switch, a mouse pointer appearing for just a second and disappearing again. After that, resolution switch after resolution switch.