Reason for change of manufacturers, beside the obvious, is reportedly inadequate Samsung's 20-nanometer manufacturing process that fails to satisfy the requirements of profitability. TSMC, on the other hand, is capable of assuming production and meeting the technical requirements, as well as requirements for production capacity.
According to the same source, the manufacturer changes for the A8 generation of chips will not affect their decision when choosing a manufacturer for the A9 generation, so Samsung could once again produce for Apple in 2015.
[Ed - The real reason for the change is that Samsung and Apple are direct competitors and Apple would like nothing more than to get away from them. We have watched as Apple has slowly pulled away from their one time partner ( like they did with IBM, Motorola and many others). Fortunately for Samsung (but bad for Apple) the move has not been as fast as normal. This has been simply becasue no one else could offer the capacity at the prices Apple wanted.]
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