I’ve looked at pfx_x and pfx_z movement on a day-by-day basis to see if there are other days with the same issue. And, yes there are; here are the plots for the games from June 26 to June 30:
The game_id 201306282 is the second game of a doubleheader between Cleveland and the WhiteSox and so I looked up the starters to see if their pitches of this day were different from the rest of their starts.
Here are Jose Quintana’s plots for 2013:
And here are Carlos Carrasco’s plots for 2013:
So, both pitchers had the same change in overall pitch movement on the same day. This just can’t be right.
Explorations continue …