When I start a drag and drop using a right click in WPF it will fire the drop on any target with out waiting for the right mouse button to come up.
To drag, the mouse needs to be captured, which can only be achieved when the left mouse button is down. See http://msdn.microsoft.com/en-us/library/system.windows.uielement.capturemouse(VS.95).aspx (for Silverlight, but I think it's similar).