it would appear that you have set the property "StartFromCurrentNode" to true or have used offset property to determine where the panel bar should start from (not that offset property is dynamic and will change based on the current page).
It seems that to achieve your desired functionality you will need to modify the SitePanelBar control a bit. The user control for this control is located at this location:
You could for example implement a new property there and name it "RootPage" and then change the databinding logic in such a way that it always starts from the page that has been designated by the end user as RootPage.
I hope you'll find this information helpful. Let us know if there is anything else we can do for you.
the Telerik team