Your statement was that there were "no other possible conclusions." Whether you find those conclusions persuasive or not does not refure the fact that there are other possible explanations!
To characterize a return to Humane Vitae as a "sudden change" also seems like quite a stretch.
To me, the most satisfactory answer is that younger married couples today are more likely to follow the teaching of the Church. This agrees with empirical and anecdotal evidence concerning Gen-X vs. the Baby-Boomers.
Honestly, I was quite startled to find the trend I did when I analyzed the data. I had expected it would confirm a collapse in the Catholic birth-rate.