On most of our 11gR2 databases that we have been using over the last months, the passwords start expiring recently.
I noticed that the default password expiry time is 180 days on the DEFAULT profile, so:
ALTER PROFILE "DEFAULT" LIMIT PASSWORD_LIFE_TIME UNLIMITED;
Or, you can create a profile for your application-owners with unlimited password life time - it's up to you.
Note: this is also a problem on your DBSNMP user in your EMREP database when you use Grid Control with a 11gR2 database for EMREP.