This is the default behavior. RadContextMenu have an EventName property that specify which event to listen to in order to open RadContextMenu and the default value is MouseRightButtonUp.
If you want to use both MouseRightButtonUp and keyboard right click button you could change the EventName to ContextMenuOpening like this:
Let us know if you need more information.
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items