Because they used to send the pitching coach out to remove the pitcher? I have a vague recollection that some teams did back then.
From what I understand (and can remember), managers did this for two reasons (at least two reasons I remember):
1. They were afraid of getting booed, which is definitely not the case here.
2. Some managers gave nigh complete control to their pitching coaches, basically saying, "You're 95% in charge of the pitchers, you deal with them. I'll tell you when to take them out." This included taking pitchers out of games, because essentially they were the pitchers' boss, not the manager. I don't think that this was done a lot, but I have a vague recollection that Rene Lachemann was one of those types of pitching coaches. Sorta like Johnny Sain before him or Leo Mazzone after him. Gammons used to write about him all the time, so I always assumed that he was well-respected and not simply Mauch's drinking buddy.
I was 12 at the time, so my memory could be off.