I'm somewhat surprised more sinker/slider guys don't try backdooring hitters with that sinker more often in 2 strike counts. Hitters are programmed to be expecting/guarding against that slider so much that anything that starts off the plate outside, they'll give up on it really early anticipating the slider. You'll get that K looking a decent amount of time, they'll roll over it a lot of the time, and it forces to the next guys to keep the sinker in their mind for the 2 strike counts, which should make the slider even more effective.
PLus, i think it was the perfect pitch. It was a strike according to Savant by the width of a c*nt-hair.