Rev, I've never heard of Word doing that before, BUT it does sound as though it may somehow have put -" (or whatever it looks like) into the dictionary/table it uses for auto-correction. The user is allowed to add items to that, and you may have done it inadvertently.
I'd open the "Auto-Correct" dialog box (probably from the Tools menu unless you're using a new version of Word with that gawdawful "ribbon" nonsense) and look in the table to see if you've accidentally added -" or --" into its list.
A few years ago, I was writing a lot of software test cases where I frequently needed to use the word "field" (as in "Enter some text in the Address field") and I could NOT stop typing "filed" instead of "field" so I actually inserted an auto-correct entry into my table such that it changed filed to field for me on the fly. It was on my work computer, and I never needed to use the word "filed" so it was worth it to save myself having to backspace and retype all the time.