It's a dispute over land. Here's a couple panels from the MK4 comic that go into it. Click on them to view them all the way:

Note that this comic features Kitana, representing Sindel and Edenia, mediating a dispute between two Outworld races, over Outworld borders, where Shao Kahn has authority.

When Tobias was writer, Edenia was what Outworld had been before Kahn took it over and that Kahn had died in MK3. Which meant there was no more Outworld, it had all been turned back into Edenia, and Kitana and Sindel were technically the new rulers of the Centaurs and Shokans.

It wasn't until Deadly Alliance that this was established/retconned that Edenia was a seperate realm that had been merged and unmerged, Outworld had always been called Outworld, and Kahn was simply in hiding, healing up.

So in this comic, the reason Kitana is arranging the treaty was simply to have peace in her lands. Whereas in Deadly Alliance, it of course had to be changed. So now it turns out she was trying to make allies inside Kahn's lands so she could wage war on Kahn and finish him off. Which is...surprisingly proactive.

Hey, thanks to all who have responded! I didn't know about the info in the comic, thanks for showing that, ]{0MBAT! It's interesting to see it over land. So wait, RazorsEdge, does this mean that they were fighting over land in Edenia as it's own realm, or Outworld?

Well, RazorsEdge701, interesting take on it. My theory is that some lands went back to Kahn (in his Outworld) but that many of them such as what Goro and Motaro's races are fighting over were either always part of Edenia and thus never really Kahn's to begin with, or, after the Edenia/Outworld merger was over, then some parts ended up where they shouldn't have. Either way this scene doesn't necessarily have to be retconned out.

The Bears and Packers rivalry should be explained further.

Some rivalries don't need a back story. Its just you're too close to us!

Go Bears!

Anyways...

I think they should have a back story. I don't quite understand why you have something going on in a game like this that hasn't been explained very much about why it's occuring in the game itself.

But yet, confusion and being in a concern train of thought is also fun...

Meh, I'm on neither side on this story.

I wouldn't mind it if were explained in a backstory written for Motaro, Goro, or some other character involved in that story. Maybe if they do fulfill my dream, and make an RPG were you play as Shao Kahn trying to make a grab for power in Outworld, you could find out more about the Shokan/Centaur rivalry?

To be honest, though, I think the chance to really follow that story is long gone now. MK4 would have been a great place to follow it up, but alas. Now I think it would be elaboration for elaboration's sake.