Probably because you changed the default apps for certain file types in your previous version of Windows and forgot all about it.
Windows 10 has set those associations back to their default status. Unfortunately, you’ll need to go through and change them all again manually.
Fortunately, it’s not too difficult to do. Simply right-click a file that isn’t opening as you want it to, highlight ‘Open with‘ and click ‘Choose another app.’ Now select the app you want to use to open the file type and tick the ‘Always use this app.’ Now hit OK.