The always well-informed Yukari Iwatani Kane and Ian Sherr, reporting for the Wall Street Journal:

Several people familiar with Mr. Papermaster’s situation said his departure was driven by a broader cultural incompatibility.

Mr. Papermaster had lost the confidence of Mr. Jobs months ago and hasn’t been part of the decision-making process for some time, these people said. They added that Mr. Papermaster didn’t appear to have the type of creative thinking expected at Apple and wasn’t used to Apple’s corporate culture, where even senior executives are expected to keep on top of the smallest details of their areas of responsibility and often have to handle many tasks directly, as opposed to delegating them.

Well, that explains it nicely. Papermaster failed with the iPhone 4 antenna but that wasn’t the only reason he got canned, just the straw that broke the camel’s back.