Why do I get notifications about new Photos memories from a decade ago?
10 year old photos must have been scanned for faces and scenes by now. Whatever new processing happens might be due to some changes or features they bring to Photos or Siri. 

As far as memories are concerned from Apple, 


  Rediscover favorite and forgotten moments from deep in your photo library. Memories automatically creates curated collections of your most meaningful photos and videos.


So (These are my guesses) whichever photos involve you in a happy state, your friends, or those shared or opened a lot would appear back.