I wonder if they go up there looking for a specific pitch on the first pitch. If they see that pitch, they'll swing away. If they don't see it, they take it. Maybe sometimes they just want to take a few pitches to see how the ball is coming out of the pitcher's hand? Just because the ball ends up down the middle of the plate doesn't necessarily mean that the hitter knew that specific pitch was coming or was ready for it. It's very frustrating though. I remember Mookie doing it all the time and NESN talking about him taking so many first pitches. Well, it worked out pretty good for him in 2018 I guess.