Is it ever explained why the Yardies end up hostile?

[Omario]

I also never understood why they try to kill you in the mission ‘Kingdom Come’. The wiki says very little either.

[Shiinyei]

According to Kenji's missions, the Yardies and the Cartel are "allies", or at least they are both in good term.
In Kingdom Come, Catalina makes a deal with King Courtney where you are supposed to pick up a stash for King C. but you end up being trapped by some drugged madmen (not sure if they are on the Cartel or the Yardies side).
You escape the trap, and now that King C. knows that you're still alive, he still probably wants to kill you.