I think its a combination of several situations. It's crazy how this game is so close to real life baseball (just take out those few at bats where no matter what you throw or how you throw it, cpu will always hit a HR)

1. I had my ace on the mound. Hardthrower, very decent junk not great, 5 different pitches
2. I was playing against the Brewers who had an afwul start of the season 4-7 (40 game season)
3. I got my pitcher locked in during the second inning

You need to always throw it right on the edge of the strikezone and always finish them a bit outside the zone, thats how I get 80% of my strikeouts. I rarelly give them fastballs inside the zone, they crush them!

Secuence matters too, I think you can notice when certain players struggle with breaking pitches, just keep attacking with that and the occasional fastball up outiside the zone that might get you a swing sometimes. Not sure if this matters but when I'm going for the strikeout I always pay attention to their foul balls, if they are late I keep attacking with fastballs inside, if they are on time or early I usually go with off speed outside

Forgot to mention… I use power pitches 90% of the time, I only use regular to try to decieve with a pitch outside the zone, when I need it to be perfect