A hotfix has recently been issued that resolves an issue where the Windows 7/Windows 2008 R2 client "forgets" its dynamic site name during the startup sequence.

The net effect of this being that the client always makes additional generic DNS queries which return non-site specific DC names back to the client. 
The DC returned from the DNS query will in turn tell the client to talk to a DC in its own site.

However, this peculiar type of amnesia will affect your systems particularly negatively if you've either configured the PreferLogonDC registry setting or else if DNS registration for branch office sites has been turned off and the link to the main site is down.

With PreferLogonDC set, the DFS server the client talks to will place itself at the top of the list (even as it sends you the site you're in) - if the client talks to the same DC again the next time it asks for DFS referrals for Sysvol the process will be repeated.

Fix details are otherwise available in KB266938.

Client computer uses site-less SRV records after you restart the computer in Windows 7 or in Windows Server 2008 R2
http://support.microsoft.com/kb/2666938

DFS-Related Processes on Client
http://technet.microsoft.com/en-us/library/cc782417(v=WS.10).aspx#w2k3tr_dfs_how_wxun