Three possibilities come to mind - you have:
1. an addin that's causing this behaviour. Running Word in safe mode (i.e. hold down the Ctrl key while starting Word) should quickly reveal whether that's the case, as all addins will be disabled;
2. a faulty Office installation. Try repairing it (via Windows Control Panel > Programs > Programs & Features > Microsoft Office (version) > Change > Repair); or
3. a document that has acquired some of corruption. Corrupt documents can often be 'repaired' by inserting a new, empty, paragraph at the very end, copying everything except that new paragraph to a new document based on the same template (headers & footers may need to be copied separately), closing the old document and saving the new one over it.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]