To reproduce:
- click a RadCheckBox with the right mouse button and you will notice the the state of the checkbox is changed.

Workaround:

public class LeftCheckBox : RadCheckBox
{
    protected override void OnMouseUp(MouseEventArgs e)
    {
        if (e.Button == System.Windows.Forms.MouseButtons.Left)
        {
            base.OnMouseUp(e);
        }
    }

    public override string ThemeClassName
    {
        get
        {
            return typeof(RadCheckBox).FullName;
        }
        set
        {
        }
    }
}