SMPlayer Support Forum

I'm afraid I have no idea. All smplayer does is provide some *.desktop files with a list of the extensions it can open. But beyond that it's up to the operating system to select an application to open them.

Perhaps there is some action taken by the "Ubuntu Software" installer service that is forcing AVI files onto SMPlayer? As a test, I removed SMPlayer through the Ubuntu Software Centre and straight away mpv player opens AVI files by default.

I guess nautilus (or the file manager you use) is somehow configured to open smplayer for avi videos. Of course if you uninstall smplayer, avi videos won't be opened by smplayer anymore.

Check out the following answer, one of your mimeapps.list files might be corrupted. I had a similar problem with KFontView getting on the associations list for every file type...