
Hirata was selling drugs, this is against the head groups policy and Yashiro rumbled him. Yashiro planked the drugs meaning the police found them, it was to make sure Ryuzaki would be kept safe from Hirata who was trying to kill him (by putting him in jail for a time as Ryuzaki was the one who was framed). Hirata was working with the Gouda group with the drug deal, and now the Gouda group leader is annoyed because he has basically missed out on a deal worth thousands of pounds and is wanting Hirata to accept responsibility for fucking it up. So he's looking for revenge and trying to kill Yashiro but also presumably has to cough up the debt from the failed deal. I think the Gouda group is an unofficial sub group of the Sanwakai group, not sure what relationship the Sanwakai group have with Hirata's head group.
My comment is based on memory though, so might be wrong about the details about this new Sanwakai group.

Also, you probably already know the backstory as to why Hirata doesn't like Yashiro to begin with, but (from memory) Hirata did some petty work for the Doushinkai group, then he took some of the profits from whatever it was that he was doing and came to the attention of Misumi who was at that point the boss of the group Yashiro is now. Hirata gets envious of Misumi's right hand man, or whatever the yakuza term is, because Kurubane is a threat in Hirata's eyes because Kurubane is not motivated by furthering his own position in the group, but is instead just devoted to his boss Misumi and at this stage has been with him for 15 years, Hirata kills him, still not quite sure why he had to go that far though .Hirata eventually becomes boss of his own group, but he isn't as succesful at making money for the Doushinkai like Yashiro, so this adds to his envy, and he ends up trying to kill fellow group members (because they are all leaders of the sub groups) Ryuzaki and Yashiro, because he failed at his attempts to import/deal drugs and the rivalry that existed before that.
Apologies for the poor explanation, I'm sure I've made some mistakes but you get the gist. He's a bad egg, but from his perspective Hirata basically had to work up from the lowest role of the group to getting leader, whereas he think Yashiro is only boss of his group because Misumi took a fancy to him, which may be partly true, but Yashiro likely wouldn't have lasted long as leader if he didn't have the respect from his team and couldn't make money for his group, apparently Yashiro's group makes the most money for the Doushinkai which is probably making Hirata feel inferior.

No problem, glad you could understand, what on reflection was quite a jumbled post! I made a post earlier which broke down the Doushinkai group if you want to know more about the members/different sub groups of that, that have been prominent so far, but now a new group relating to Hirata's drug deal has been introduced the Sanwakai so it seems a bit confusing to have all these new characters introduced after so many chapters of the Doushinkai group.
Okay i dont get a thing whats going on..