I was watching a YouTube collection of some "heated" plays (where people got ejected or the benches emptied) and quite a lot of them involved a batter hitting a home run, and the fielding team starting a fight over the batter staring too long at the ball in flight. The announcing teams made it obvious that "looking at" a home run was considered poor etiquette, but not really why.
The batter standing at home, or jogging slowly to first, doesn't seem to me to be interfering with any kind of play, nor does it seem to be particularly rude to admire a well-hit ball. (It's not like these batters were being overtly insulting or aggressive towards the pitcher.) If anything, the batter is doing himself a disservice if the ball ends up in play -- he's wasting time that could go towards extra bases.
Where did this element of baseball etiquette come from?