Why did the photos app auto-create a random photo album?

Solution 1:

The automatic album creation is a new feature:

The Photos app creates albums for you from groups of related photos.

(Source: Microsoft documentation.) It didn't add all the photos taken on that date because it thought some were too similar to others:

Not all of the related photos will be in each album. If you have several similar photos, it’ll select what looks like the best one.

Date and time are definitely taken into account when grouping the photos; a CNET article suggests there might be more complex logic too:

Here, you'll find albums that the Photos app creates for you automatically, based on date taken, location, and possibly facial recognition (people in the photos).

The location would be determined from information embedded in the picture by the device that took it.

Concerning the uninvited appearance of the app: That seems to be a bug experienced by many people with the management of Universal app processes. For some, it's caused by a certain version of DisplayFusion.