Possible influences:
Last rounds of league expansion may be a factor.
David Stern may be a big factor. Between late 80s and early nineties lots changed in NBA averages.
90-91, first time 3ptas over 7 per game. Up to 15 in 5 yrs.
86-87, first time 3pt fg% over 30%. Up to 36% in a few yrs (rim tightness/ uniformity?)
89-90, last time teams avg'd 28 ftas.
93-94, last time teams avg'd 16 TOs.
93-94, last time teams avg'd more than 8.5 steals.
91-92, last time teams avg'd 14 ORs.
92-93, last time avg. pace was above 94.
Stern thru rules and management of officiating had impact on some or many of these trends.
Changes in coaching and athletes play a role too.
Did home court win% change? Yes, big time.
http://espn.go.com/nba/story/_/id/12241 ... ge-decline
When did teams go to charter jets generally?
More active medical staffs?
Superstar calls?
When was the first modern era salary cap? 84-85. 89-90 big jump to over 10 mil.
Expanded TV coverage? (did best / big market teans win more?) Local cable revenue not shared?
Sports center? The beginning of Jordan era NBA culture? Mega mega stars? Change in frequency of chuckers? (More than twice as many guys over 15, 18 fgas per game per team in 1978-9 than last season. Median efg% about the same but distribution shape may vary beyond that measure.)
Just throwing out possibilities.
74-90, more player partying and / or saving it for the playoffs?
Bigger & bigger bucks coaxing more of the talent edge out of players hungry for the rapidly expanding available dollars?
More and better video work and the slow rise of analytics by best teams?
Exlansion in underclassmen? Expanded roster sizes?
Rise in big salaries for top coaches?
More emphasis on defense by top teams?
Less white players?
Change in pay level / quality of officiating?
Change in rate of game fixing influence?
http://www.sloansportsconference.com/?p=12728
Lots more parity literature to check for clues.
Shorter shot clock?