Intel's tablet drop
Quite expected if you paid attention. Intel had a "contra revenue" program where OEMs paid full price for their tablet SoCs, and then Intel refunded them MORE than what they paid as a kickback. As one wag said, basically they're giving away their tablet SoCs wrapped in dollar bills. They didn't do this for the 'real' x86 CPUs used in Surface Pro, only for the SoCs designed specifically for the true tablet market.
Obviously when you slow or stop that program, companies that were buying x86 SoCs for tablets (other than Surface Pro, which is really a laptop in disguise) because they were 'free' went back to ARM SoCs. Thus the large drop in tablet revenue, even with Surface Pro sales staying constant or even increasing.