Just a possibility, you might want to check to see if your SQL user accounts have a password expiration policy set.

select name, is_policy_checked

from sys.sql_logins

(shamelessly stolen from this thread: http://www.sqlservercentral.com/Forums/Topic397068-338-1.aspx)

edit: just wanted to mention that I think this check is for SQL server user accounts that are NOT windows logins)