I doubt that anyone can help just with that description. This is an extremely strange behaviour, one which I have never observed in ten years of working with Alfresco systems integrated with Active Directory. I can also not think of any reasonable cause / trigger for such a behaviour within Alfresco.
Did you only configure the LDAP-AD integration, or did you also configure Kerberos / passthru for authentication? What is the authentication chain configuration? Ideally, it helps in such situations if you can provide your full configuration (wihthout passwords / sensitive infos of course - but best replace them with xxx instead of not providing those properties).