I first played D&D when I was 5 ('84). I used the red box set, and I was the fighter. My friend and I did not play for very long, but I was hooked. I didn't play again till 2nd edition in high school.
1980 just happens to win here out of the batch of years from about 1978 to about 1983 when so many of us got in for the first time.1980 for me. Huge year for these parts.
So, what is significant about 80, 88, 92, and 2000 to cause the peaks. I have my theories, but what else?
OK, some of the 'peaks' are only minor.