I personally would have attributed that to the drift of the downstream effects:

  - industrialization ultimately allows for women to meaningfully enter the workforce, which is a major pressure on fertility. This entering of the work force is quite delayed due to legal, cultural, and training issues.

  - People probably don't just magically know when to have fewer kids, and I'm imagine the ramp-down happens over time.

  - More specific things which suppress fertility might happen even later in the cycle. (as just one example: the accident of requiring car seats requiring the purchase of a much more expensive 3rd row vehicle, and so disincentivizing)

Is that along the lines of what you were thinking? I'm curious for what you think is causal here. (I'm not an expert, and am not trying to claim any expertise!)