Active directory grouping issue

I have this case, user was suddenly added to one Active directory grouping. When user login, a logon script takes effect. The user then was able to access other department shared folder which is not supposed to be.

Ex.
AD Grouping Shared folder

Group Good = T: \\server\gooddept
Group Bad = U: \\server\baddept

OS Server: Windows 2003
Files Server: Windows 2003 clustered
OS User: Windows Xp

User 2 suddenly acquires the group membership/profile as User 1. We have check with the admin and they said nothing was changed. They were saying there’s a problem with the Active directory.

How to investigate and what are possible issues related to it?

Thanks.