Unintended format change

I have a fairly large (10,000 kb) spreadsheet with 20 tabs.  I've been using it for years and save a backup copy every night.  When I opened the spreadsheet today, the formatting of many of the columns on about half of the tabs had been changed (different colors, column width, currency to date, etc.).  I did not make these changes and I almost never access some of the tabs that were changed.  Both the main file and backup had the same changes.  Any insight would be appreciated.