Modern versions of sendmail like the one in OS X separate the handling of mail coming in and mail originating here to go out. The first line starting up sendmail is the one that listens on port 25 for incoming mail and handles it. The second line starting up sendmail with a different configuration file waits for messages originated on the Mac and sends them out. Because of the "q1h" argument, both of these run as daemons and process the queue once every hour. The first one uses /var/spool/mqueue as its queue directory; the second uses /var/spool/clientmqueue.
I have had the same crash and agree that it was due to /var/spool/clientmqueue not being emptied. But looking at the crash log, the impllication was that the error occurred while processing the receipient name. In my case, the receipient name was "root" as discovered by looking in the qfXXXXXXX file in the queue directory for the line that begins with "R". I changed the recipient from root to my email address and the queued message was sent out the next time the queue was processed.