There has been some talk recently about low home run rates in October being attributed to a de-juiced ball. I don't have any data on this claim, other than, allegedly, fly balls are travelling about four feet less in distance that the regular season ball.
I'm not sure how this would translate into home run totals, but there are a number of reasons why October HR totals would be lower simply because it is October. First, during the regular season every team plays every other team in the league. By definition, the average lineup faces the average pitching staff over the course of the season. For a playoff team, their lineup would face less than average pitching because their pitching is most likely above average and their lineup doesn't have to face their own pitching staff. But in October only the top third of teams play post season ball. Each lineup would face the top third of pitching staffs. As an example from this year, the Yankees set an all time record for HRs against another team, the Orioles. And the Orioles pitching staff set an all time record for giving up the most homers in a season.
But none of the Orioles pitchers are in the playoffs, so the Yankees won't be able to hit homers off of them. Additionally, playoff series have travel days during series and between series. And if a series does not go the full number of games, there are additional days off. Playoff teams can delete their least effective starters: their number five starters. If a team sweeps their division series, they can reset their rotation to their ace for game 1 of the LCS, skipping their number four starter.
In summary, only about the top 20% of pitching will be faced in the post season. This is why home run totals drop in October.
