I originally thought it was because he wanted more money, technically yes, but it was the original amount promised when he first signed on the first one, because of it's success, and especially RDJ salary, they took his pay and reduced it to pay to RDJ. They labelled him a diva, and his career took a serious hit.
I heard that initially, but his original contract was for a lot more and they decided to change it, personally i think Don Cheadle was a better Rhodey as how Terrance talks is really annoying
Besides, all reports I heard indicated he did go diva about pay and billing.