The code in question was reading an XML Document via http.
httpQueryString = "http://coreivfdev.sspf.xxx.xxx/servi...oreService?PIN=" & GetUserPin & "&Application=Test"
If XMLDoc.Load(httpQueryString) Then

This would hang Excel and not permit a double clicked document to Load.
I found this workaround using Application.OnTime to provide a few seconds for any double clicked documents to open.
Interesting that neither sleep nor DoEvents would allow a double clicked workbook to open during their Idle period. Neither works as advertised.

Private Sub Workbook_Open()
'Populate the XML fields
'Use Application.OnTime to give Excel 5 Seconds to Open any Double Clicked Files at Startup
Application.OnTime Now + TimeValue("00:00:05"), "PopulateOfficeXMLTags"
End Sub