What's Up With Punt Distances?

As part of an effort to improve the realism of historical replays in Football Mogul, I recently calculated the average punt distance for the last 72 seasons:

There is a general upward trend, which I think can be explained by improvements in technique and training (similar to the improvements we've seen by field goal kickers). But there are a couple things that I can't explain:

1. A sharp but temporary increase from 1959-1965. The average punt during these 7 seasons was 42.6 yards, about two yards per punt higher than the previous 7 seasons. It then dropped sharply in 1966, dipping below 40 for most of the 1970s.
2. A drop from 43.4 to 41.9 between the 1998 and 1999 seasons. It took almost a decade for the average punt length to get back to the level it was at in 1998.​

I can't think of any rules changes or shifts in strategy that would cause these changes. If you have any theories, let me know on Facebook or by commenting here.