This is pretty much what went down.
Overeem got told the next round of the tournament would be in October so he verbally agreed to that. Then, a few weeks later he logs onto a social networking site to see that there is an official card set for September with him fighting on it. This is how business is done nowadays. If you’re going to make a big professional decision about an employee like fire someone don’t call them or send them an email or fax just post it on twitter or some other website that you don’t consider the internet. This obviously upset The Reem and he communicated his displeasure.
Overeem has one fight left on contract with Strikeforce. This means if or when he won the next round of the tournament his contract was up. Not too smart on the end of Strikeforce when setting this whole thing up. So Overeem was upset about the date being changed and Zuffa figured why not just scrap him from the tournament. This way if he did win the next round and his contract was up he wouldn't have crazy leverage in the contract negotiations since he would now be in the finals.
What Overeem should have done was wait to negotiate his new contract until after the next fight, assuming he beats Big Foot, then he could demand huge money to fight in the tournament finals. Instead he complained about a month difference and called Dana Whites bluff and now only has the leverage of being the #3 ranked heavyweight in the world which still isn’t bad.
The whole situation sucks bad but both sides fucked up here. Zuffa uncharacteristically got greedy; I know it was bound to happen sometime. Strikeforce didn’t sign him for the whole tournament. Overeem called the bluff and tried negotiating tactics one fight too soon.