The population of authors writing about overpopulation exceeded the carrying capacity of the industry, and experienced a Malthusian catastrophe beginning in 1975.

On a more serious note, the 1968 release of The Population Bomb correlates with the peak of the Ngram acceleration. The steepest increase in interest begins in the 1968-1969 period. This book was highly popular, but it did not cause the interest overall (it comes prior to the peak, but well after the initial, slower ramp up).

This peak follows the tail end of the Green Revolution, when new disease resistent and high yield crops were developed and implemented around the world. 1974 specifically was the year that India became food-independent. It's hard to say whether this was or was not a coincidence, but one of the points laid out in The Population Bomb was that India was on the path to its own malthusian catastrophe.

The decline in interest feels more like a return to the norm. After the 1974 peak, interest was still fairly high in comparison to the 1930s-1950's, it's just that the scenarios outlined during the peak of concern did not appear to be coming true, or at least not at the pace predicted by something like The Population Bomb.

The preface to the initial publication was:

The battle to feed all of humanity is over. In the 1970s hundreds of millions of people will starve to death in spite of any crash programs embarked upon now. At this late date nothing can prevent a substantial increase in the world death rate

Here's a thread we've had before that may be of interest, see specifically the top answer by u/flyingdragon8 . This discusses the Green revolution and what was going on in the world of agriculture immediately prior to the peak you're talking about.