AD Locking Users Out Automatically

I have a few users that are constantly locked out of their PC when a certain server is turned on. We have narrowed it down to just one server. They will never be locked out if the server is not turned on, but we all know that we need that server on at some point or another even for communication purposes. I can go into AD and unlock their account, but dont have a reason why it is only happening when that certain server is turned on. Any help would be greatly appreciated. Thanks