To reproduce:

Press the down arrow to open the drop-down, then click and hold the scrollbar to move it down. If the mouse cursor strays off over the scrollbar (e.g. left), when you let go of the mouse button the drop down closes. This means you have to ensure you keep the mouse cursor on the scrollbar.

Workaround:

this.radDropDownList1.PopupClosing += radDropDownList1_PopupClosing; 

private void radDropDownList1_PopupClosing(object sender, RadPopupClosingEventArgs args)
{
    RadDropDownListElement el = sender as RadDropDownListElement;
    if (el != null)
    {
        if (el.ListElement.VScrollBar != null &&
           (el.ListElement.VScrollBar.ThumbElement.Capture ||
           (bool)el.ListElement.VScrollBar.SecondButton.GetValue(RadButtonItem.IsPressedProperty) ||
           (bool)el.ListElement.VScrollBar.FirstButton.GetValue(RadButtonItem.IsPressedProperty)))
        {
            args.Cancel = true;
        }

    }
}