Why do all of the files in /etc get changed every once in a while?

I am rolling Tripwire out to our linux RH boxes (3, 4, and 5) now that
all of the Sun Solaris servers are done.  Several times now on a monday
morning Tripwire reports that every file in /etc has changed on a couple
of the servers.  Its always the timestamp.  It is always a change to the
same date and time for every file in /etc.  This has happened on RH3,
RH4, and RH5 servers.  It only happens on a few each week.  Why would
this be happening?

-- 
Christopher Barnard
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