To reproduce:
radPopupEditor1.PopupForm.VerticalPopupAlignment = VerticalPopupAlignment.TopToBottom;

Workaround:
private void RadPopupEditor1_PopupOpening(object sender, CancelEventArgs e)
{
    var args = e as RadPopupOpeningEventArgs;
    args.CustomLocation = new Point(args.CustomLocation.X, args.CustomLocation.Y - (radPopupEditor1.Height + radPopupEditor1.PopupForm.Height));
}