"The author would like you to open this as read-only"

A few days ago I started getting this message whenever I open a certain file:

The author would like you to open this as read-only, unless you need to make changes. Open as read-only? (Yes/No/Cancel)

I looked up the message and learned how to make it go away. My question is: What in tarnation possessed Word to start displaying it? I'm the author of the document. I never asked Word to display this message; in fact, until today I didn't know it was an option in the Tools dialog, and had I ever opened that dialog.

No one else set the option, either. The document is under source control. No one but me has committed it to the depot since I created it on 2/21/2019.

Word appears to have enabled this option all on its own. I really wish I knew how to make it stop giving me little surprises like this one!