Based on my experiences, two thing come to mind.
First, is the pitch inventory that a man has to work with, and that inventory's workability (dependability) on any given day. Second, how deep into the game (inning) has he worked.
The first consideration has a lot to do with each pitch's effectiveness and that effectiveness of each pitch that a man feels comfortable with during his appearance. Say his "go to pitch", for whatever reason(s) is his slider, and that has a high degree of workability during his appearance, then his overall "reach" into his bag of tricks is going to be a slider.
Even when predictability is evident by the other bench, dealing with a pitch that's really good is a hard nut to crack. I've had pitchers that'll deal nothing but junk all day/night long and survive rather well. Each pitch was as predictable as it comes - dealing with even a two pitch repertoire - curve ball and "floater" was another story all together.
The second consideration gives way to fatigue and a certain ..."how ya feeling there sport?" Pitchers are not machines that have that .. plug in and go feature. They're human, with all the human traits, ups and downs just like anyone else - only worse. Attitudes, mood swings, heck even the slightest look the wrong way can set these guys off like a bottle rocket ... pow, right to moon Alice kind of stuff. Also, the deeper into an inning a man goes usually warrants a careful look-see into .. how to get-em out of there, and quickly. Some guys are their own worse enemy. They get this insecurity complex after facing nine guys in one inning ... go figure. Even worse, looking over their shoulder trying to get a glimpse at the bullpen to see if anyone is "up", can rattle a guy to no end.
I like your question. It shows depth and reasoning not found, usually, in the amateur ranks. Very deliberate and complex, but worth considering in greater thought. I'd like to see more of your questions orbiting the same thought process.