Please refer the Original Post and KB article link here
Symptoms
Some computers may reboot unexpectedly after they are removed from a collection in System Center Configuration Manager 2007
(ConfigMgr 2007) when that collection has a maintenance window.
Cause
The problem occurs because the maintenance window is removed after the computers are removed from the collection.
If the assignment was downloaded successfully and was triggered, then once the maintenance window was removed before the policy of assignment was deleted, the assignment will still be installed. It means that there were 2 important policies that arrived at the ConfigMgr 2007 client in this issue after removing the computers from the collection:
(1) Maintenance Window delete
(2) Assignment policy delete
The reason why the issue occurred is that the policy 1 arrived before policy 2. Based on current ConfigMgr 2007 design, the program policy deletion is not prioritized over maintenance window policy deletion because the policies were evaluated and applied one by one and there is no priority flag in the policies.
Resolution
This is by design behaviour in ConfigMgr 2007.
More Information
In the current version of ConfigMgr 2007, we need to pay attention when moving computers between collections that have a maintenance window setting because the maintenance window setting means that the computers cannot perform change outside of the maintenance window.
It is recommended that administrators follow the best practice of naming the collections in a specific manner (such as “MW-“), then the administrator know the collections that have a maintenance window and thus will not remove operations outside of the Maintenance window.
http://technet.microsoft.com/en-us/library/bb694295.aspx
Best Practice Recommendations
Maintenance windows are not intended to function as a primary scheduling method for programs. Instead, they are designed to limit the interference of software deployments or changes with critical system functions. For example, one likely use of a maintenance window would be to specifically restrict system changes on a collection of client computers to the period between midnight and 2:00 a.m., when network traffic is at a minimum or when those computers are not engaged in other activities.
It is recommended that if you use maintenance windows to restrict system changes, you should create specific collections for this purpose rather than use the default collections. Additionally, naming these new collections in a specific manner (such as “MW-“) can be helpful. One or both of these methods will help you manage your collections more easily and effectively.
For the most current version of this article please see the following:
Does this still apply in 2012?