I can't offer any explanation but I've just discovered that having linked geography data types anywhere in an Excel file makes Word crash when opening a mail merge document based on that Exel data source. Took me a while experimenting to find out why my reliable pairing was crashing but converting linked data types back to text solved it.
I hope this helps someone and would MS like to look at the cause.
There's also an issue with linked geography data to UK postcodes but that's another story.....
