Can someone explain me this competitors hack?

Hello everyone! I want to launch a tech blog and service and during analysis of competitors backlinks I found the next hack (or its just a normal thing, so I dont know).
Every competitor has tons of backlinks from different spam forums with posts full of backlinks (up to 30-40 on 1 page) to different websites. And all these posts from different forums are created in such way that they just refer to each other thus helping to improve its DA and PA.
Is my theory right? Please check some examples, I just cant understand how this can be possible.

https://bit.ly/2FfeJV2
https://bit.ly/2CaGEkM
https://bit.ly/2LZDBQR

There are 3 examples with:
1) group of links (http://prntscr.com/m4fqu4) where one link is related to competitors website, while others are related to similar websites
2) group of links (http://prntscr.com/m4fr54) to another forums with different topics in the same niche
Please help.

Not a bad idea, what they are doing
They have created whole forums just for link building and external linking
Definetely not what Google wants