It's a complete mystery
I deleted your other post by the way, thank you for alerting us to it. For some reason, may be copy/paste, HTML formatting is added. The engineers are working on it.
This thread is full of examples the same as yours, and when I Google, " Open Documents Suddenly Become Read-Only ", I see lists full of the same problem, but with no definitive solution that I can find.
I don't even have a workaround to offer you because the Google examples as well as the options given here all seem to work or don't work, intermittently.
Perhaps a couple of ideas though;
1] Delete Word's Normal.dot file. When Word is re-opened it will recreate it, but beware, if you have modified Normal.dot with your own Word formatting, that will be lost.
2] Open this document from within Word. Then open a New Document. Copy/Paste the contents of the existing document to the blank one. Save this new document to some other folder then test over a period of time.
I don't guarantee anything as I am as mystified about this as everyone.