Im betting new technology across the board for APS-C.... Better ISO performance and better live-view / movie focus for all APS-C.
Space is not infinite in factories, there is only space for so many fabrication lines.... If they have a new/smaller fabrication process it took a lot of time and money to set it up, and they had to keep the old APS-C fabrication line up while the new one was installed, tested, debugged, and the resulting products tested. If they have the new one up and running, my bet is that they are ripping out the old one, changing it/converting it to do the same thing for the FF fabrication, and that means no more old APS-C chips for existing cameras.... and they will soon be replaced/discontinued.
And why APS-C first and not FF? Because smaller chips are easier to make, and there is a lot more money to be made from APS-C than FF.