If the document contains fields that look like «SomeField» or, if you press Alt-F9 and see fields like {MERGEFIELD SomeField}, that suggests it is a mailmerge main document, for which you should get the mailmerge prompt when you open the document - or maybe Word is just reacting to the presence of those fields. The only other thing I can think of is an automatic macro in the document that, for whatever reason, activates the mailmerge pane.

Just to eliminate any possible Word glitches, go to 'Programs & Features' in the Windows Control Panel and, using the 'Modify' option, repair Office.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]