To reproduce:
1. Create a new Form
2. Add a Scheduler control and set Dock to Dock.Fill
3. Run the form
4. Add an appointment to the scheduler
5. Right-click the newly added appointment and start dragging it to move the appointment
6. Let go of the mouse button to drop

You will notice that the appointment will not be dropped and the context menu will show. 

A correct behavior should be chosen.

Workaround:

Stop the appointments from dragging with right-mouse:

MouseButtons lastButton;
void scheduler_MouseDown(object sender, MouseEventArgs e)
{
    this.lastButton = e.Button;
}

void DragDropBehavior_Starting(object sender, RadServiceStartingEventArgs e)
{
    e.Cancel = lastButton == System.Windows.Forms.MouseButtons.Right;
}