The behaviour might be caused by document corruption, a rogue 3rd-party addin or by other software on your system (e.g. a faulty AV program). It might also be caused by a faulty Office installation. So, assuming it's a PC:
First, try repairing the Office installation (via Windows Control Panel > Programs > Programs & Features > Microsoft Office (version) > Change > Repair).
Second, if repairing Office doesn't fix the problem, try running Word in Safe mode. If the problem goes away, a rogue 3rd-party addin is almost certainly the culprit. Start Word normally, then disable all 3rd-party addins, re-enabling them one at a time until you find the rogue one, which you should disable or replace.
Third, if neither of the above resolves the problem you can't reproduce the problem in other documents, it's possible the document 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.
Failing all of the above, it would appear the problem is caused by some other process running on your system. If the problem has only emerged recently, check which programs have been installed/updated recently and check the makers' sites and other forums for issues relating to that software.
