Fellow MVP and work colleague, Tom Pacyk, wrote about a problem we've seen with Exchange 2010 SP1 UR3 where the quorum configuration does not change back to Node Majority after all odd numbered DAG members are online. You can read Tom's article, File Share Witness and Datacenter Failback, here.
This seems to occur when the alternate file share witness (FSW) is invoked during a complete datacenter failure in the primary site. The DAG quorum configuration switches to Node and File Share Majority in that case, but does not switch back to Node Majority automatically for some reason when the primary datacenter comes back online.
As Tom writes, the fix is to run the Set-DatabaseAvailabilityGroup DAGName cmdlet without any other parameters. Exchange realizes that the quorum configuration is not correct and fixes it.