A family of Microsoft word processing software products for creating web, email, and print documents.
There is only one Word setting that could contribute to such behaviour - 'Automatically update document Styles'. You'll find it under Developer|Document Template>Templates. If that option is checked and the referenced template:
• exists on the recipient's system and contains Styles used in the document and those Styles are defined differently than they are in your copy of the document; and
• either does not exist on your system or exists on your system but your copy lacks those same Styles,
then it is possible for the behaviour you describe to occur without user or macro intervention. The simple solution in this unlikely scenario would be to uncheck the 'Automatically update document Styles' option.