I don't know how many people do this manually, but editing any header in a design table causes excel to think it's an email address, so it gets converted to a hyperlink.
Is there any way to stop this happening by default? It usually takes two selections to tidy up, one to remove the hyperlink, one more to reset the formatting. Unless you accidentally click the link, then you've got email or web browsers popping up too.
While in Excel, go to Tools > AutoCorrect Options > AutoFormat As You Type and deselect Internet and network paths with hyperlinks