AD Domain Trusts

Hi, I have two new AD servers installed and running,
We also are part of our Global Network so we have outgoing and incoming trusts with our corporate office which are established and setup (ad domains and trusts)

We reached the point where are old(known as OLD from now on) AD server was no longer required, as the two new ones were up and running.
I powered down OLD and system was running fine, the following day users reported they could not access files on othe corporate server.

I powered on OLD and users were able to access the files.
What is OLD doing that the New PDC is not doing?????