This occurs when using the most recent squid from the yum repo; squid-2.4.STABLE7-0.73.3.legacy
This is a known prob and has been reported to Redhat and they have released squid-2.4.STABLE7-0.73.4.legacy as a fix.
Check the /var/log/squid/cache.log for evidence of multiple shutdown/restart cycles. I have also noticed that the logs often get very large and may also stop squid running. If you find very large logs then delete/rename them and create new empty ones (with the correct ownership/perms) prior to installing the new rpm.
See
http://bugs.contribs.org/show_bug.cgi?id=955 for details and the link for the updated file.
HTH