Google always tests pages at random this is how the system works and sees how it works. It’s all about ctr and if people are happy they move you down. Your site may be the newest on that topic and on point. So whatever your doing keep it up. Fyi you don’t have to always have a high DA or links to rank. I have noticed this same thing with newer sites more and more. I think sometimes they see older sites that maybe look like they tricked the system. I have this one competition that his work is garbage but straight to the point short. All the things we believe as SEO’s think we should be doing and beating and he is there. Other than CTR there is no reason he should rank unless he has some secrets i don’t know about. You need to have Google god watching your back.