There are a lot of hetalia dj that already exist all over the internet, I think that ppl are just uploading it all at once on this particular site
You understand the time at which a manga is uploaded to this site doesn't reflect at all on when the mangaka actually released it right? It doesn't even necessarily reflect on when it was translated.
Do the manga artists just get together and decide to come out with a ton of DJ's of a certain couple at once?