Well, he hates Dirk for one reason:

When Barkley played for Phoenix in 1996, and Donny Nelson (current Mavs GM in all practical purposes) was an assistant coach, Barkley went over to Europe to scout players for a random reason. He saw Dirk and loved him to death. Dirk told Barkley that he would be going to Auburn in the US. Barkley came back and told Donny Nelson how great Dirk would be. The rest is history, and because Dirk didn't go to Barkley's alma mater, he chooses to criticize Dirk in every aspect of everything.

He's always hated the Mavs, even when they beat the Suns shorthanded, he said, "The Mavs should take nothing from this. All this does is show how overrated Phoenix is." We can't catch a break.

And yes, dis the Mavs and become my enemy for life. See: Dingo Jellybean.