If you have attached ContextMenu(RadContextMenu) to an element and you have added logic to drag this element with DragDropManager, when you open the ContextMenu and click on the element, a new DragOperation is started (it should just close the ContextMenu). Available in LIB version 2016.1.208, it will be also available in the 2016 Q1 SP1 Release.
This is still happening more than a year later and in the latest version of UI for WPF at this moment.