Microsoft has been extremely aggressive in trying to get people to switch to Windows 10. One reason is they have also been aggressive in setting the 'default' programs people use every day (browser, email, photos, etc) to Microsoft programs (allowing them to collect more data, earn more money, etc). Even when people have set their own defaults, Microsoft has repeatedly 'reset' the defaults back to Microsoft programs 'by mistake'. This is another instance of this. So if suddenly your default browser, email, photo, maps, and/or music application switched - this is why. Switching back can be done in Settings.