After an update Windows 10 is 'restoring' the default application to its own pre-installed apps. This means that e.g. when previously using the popular Irfanview as the default app to open photos, Microsoft 'restores' its own Photos app as being the default. From then on, photos are opened with that app instead of Irfanview.
The Inquirer reports that an update released this February named KB3135173 appears to be the culprit. An user reported to the site that when he deleted that update, it was reinstalled and that the default app was again reset to the Windows 10 default Photos app. Windows 10 did inform him as it showed a 'A default app has been reset' notice.
Other users saw Internet Explorer 11 being reset to the Windows 10's default browser Edge.
Default apps in Windows 10 are different from previous Windows versions. Microsoft has made it harder to change to change default apps. Where previously software could easily set itself as default after the user gave permission, now additionals steps are required. Users need to go to 'Settings' -> 'System' -> 'Default Apps' and from then the easiest option is 'Choose default apps by file type'. This is much more complicated, especially for less tech savvy users.
It's likely that in this case resetting the default apps is by accident.
