Just a look at random dates in various Septembers.
Mon 9/21/70 10,659
Mon 9/18/72 8,608
Tue 9/18/73 15,305
Tue 9/16/75 16,025
Tue 9/21/76 20,084
Tue 9/25/79 20,336
All were dates in September where the Reds were in 1st place. Since the figures did rise in later years, I would suggest that a decade of winning during the 70's, built up enough of a base to see higher figures during September games. I should note that all of these games were against clubs that were way down in the standings, but it does show that even some very good teams didn't always have stellar attendance after school started.
Certainly there could be a multitude of factors going into these numbers and it's just a quick check, so they obviously don't compare to the entire schedule of each of those season's Septembers.