Like Randy said, this is probably a Rep Payee, and in such case, the person who is the dependent is the "owner" and the other person is simply the user acting in a fiduciary capacity.
At my institution, we treat these like pseudo guardianship's (only applies to this account, and not the entire relationship however) and the ward may be exempt from CIP depending on your institutions policy, in which case that might be why the SSN is missing. Either that or the relationship preceded CIP and your old policy didn't require SSN for account owners at that time.