There is a lot more to this than you think.
He refused to travel to America to do a press conference and DANA pulled him from the card. He then ups the stakes by threatening to retire.
Why would they have a dispute? Money of course. Conor pulls in the numbers and probably wanted a certain amount for him (likely 10m+) and maybe Diaz was demanding a big cut he didn't want to give.
Either way it sucks and I've no interest in UFC 200 now.