Sounds like the accounts were expiring.

You can checked for disabled, locked, or expired accounts using this script. You should probably monitor for this kind of thing.

SELECT sl.name,sl.is_disabled,sl.is_expiration_checked,sl.is_policy_checked,sl.modify_date

,LOGINPROPERTY(sl.name,'IsExpired') AS IsExpired,LOGINPROPERTY(sl.name,'DaysUntilExpiration') AS DaysToExpiry

,LOGINPROPERTY(sl.name,'IsMustChange') AS IsMustChange

,LOGINPROPERTY(sl.name,'BadPasswordCount') AS BadPasswordCount

FROM sys.sql_logins sl