My observations are:-
1) I have been working on the word document for 2-3 days and it includes tables and figures with captions
2) I prepared another table in Excel and copied and pasted the table into the document
3) I selected the table and tried to insert caption (right mouse click)
4) The caption was inserted, but the document was 'protected' - clicking anywhere in the document returned the cursor to the start of the document. The cells in the table appeared to still be highlighted after the caption was inserted - and I could not edit or modify the table.

In the review and document info section, WORD said that the document was protected and that i could only fill in forms.

The option to change or modify the protected areas was not available / greyed out.

Clicking anywhere in the document put the mouse at the start of the document.

Using version history, i opened an earlier version - and tried to insert the table again, and add a caption - but got the same result.

I shut down word - created a new document - copied the table into that document (without keeping the source formatting) and inserted a caption. I was then able to edit the content of the table, but nothing else. The option to insert /delete rows in the table was not available after inserting a caption. This was repeatable (once the problem occurs it persists - until the machine is restarted) Clicking outside the table cells - put the mouse at the strat of the document

After restarting the machine, I have been able to to insert the table in a new test file, and add a caption - by highlighting the table and using a right mouse click. I noticed that the highlighting of the table ended when the dialogue box for the caption appeared.

Previously, the table remained highlighted, and appears to be converted into a form.

I am not familiar with 'protecting parts of the document' - but have not been able to remove the protection in corrupted files.

I agree this does not make much sense - but these are my observations.