Apparently, "exiting" happens in the DoConfigure function which logs
to stderr rather than the log file. So you will need to arrange to
capture stderr for Xorg to get another lead to the exiting.

One way to do that is to (temporarily) move /usr/bin/Xorg to, say,
/usr/bin/Xorg.real, and write an executable text file as /usr/bin/Xorg
with the single 2 lines:
----
#!/bin/sh
exec /usr/bin/Xorg.real $* 2> /tmp/X-stderr-$(date +%s).log
----

By that, any stderr output from Xorg.real will be written to the
special log files /tmp/X-stderr-*.log where the * part is a "Unix
epoch seconds" timestamp, which would be near in time to the creation
time of the associated Xorg.0.log file.

Ralph.