WARNING: Speculation ahead, not rumor.
Before the switch to Intel chips, Apple relied on RISC processors developed by a consortium including Motorola and IBM. There were troubles getting the G4 going and even more significant troubles with the G5 that left them with a period that had G5 desktops and G4 laptops and trouble with each. Intel was the only partnership that made sense: they offered chips and economy of scale. However, Intel is now proving unreliable: they have had massive delays in chip production, Thunderbolt never took off the way it was hoped to, and now USB 3.1 is having problems (due to third parties).
Apple also signed some deal with AMD which is believed to be about the Polaris GPU- but what if it is Apple holding on the Mini, Pro, MacBook Pro, (and let's add the almost year old iMac)... Holding for AMD Zen chips in response to Intel's issues?