Are the refs sitting on their whistle more or are players not committing as many fouls?

The season charts show the season averages for various foul-related stats since 1996-1997. Season averages are computed by averaging a stat over all the games played during a season. Last season there were over 5,500 games so the data set is large.

Two interesting points about these charts. First, they are very consistent. You don’t see large disparties from year to year. Second, the number of fouls, disqualifications, technicals and ejections have been on the decline for the last 4 or 5 years. Are the refs sitting on their whistle more or are players not committing as many fouls?