I have never been able to keep the names of all the various heresies straight, so I'll take you word that "semi-pelagianism" is the heresy that unbaptized babies go to heaven.
With the wide-spread cancer of abortion plaguing the Western world since the 1960s, the actual, physical deaths of thousands of babies a day was no longer a remote, theoretical issue. It wasn't confined to pagan lands in remote Third World locations they only read about in National Geographic. "Christian" lands were now seeing, literally, thousands of infant deaths every day.
In addition to this reality, Western society was prospering in the aftermath of World War II and technological advances meant that there were relatively few people who had to struggle on a daily basis just to survive. The death of an infant (who was "wanted") was fast becoming a rarity rather than an accepted and normal part of "man's struggle to survive". Large families also became the exception rather than the norm. People found that they had much more leisure time (as all of us who post of forums have much of) and they began to contemplate their soft lives and the reality of abortion.
Couple all of this with the general loss of faith by much of Western civilization and people started asking questions about the fate of these aborted babies (and, by extention, all infant deaths). Priests and bishops, who were also experiencing a loss of faith, could not reconcile their human understanding of "love" and "compassion" with hell for those "innocent" souls: the babies who died in the womb or before baptism. At the same time, Catholic theologians and clergy who, during and after the War, had been forced together with the clergy of heretical sects and Jews were hearing the constant drumbeat that Christianity somehow caused "The Haulocaust" and were ashamed of Christ. They began to work out a theology that could eventually allow for everyone, except, possibly, the worst of the worst, to achieve salvation by exaulting "invincible ignorance", the "anonymous Christian", "Baptism of Desire" (in an extreme and perverted sense), etc. Rather than speak the truth, they salved the consciences of people who questioned (either because they had been involved with an abortion, knew someone who had been, or was just a week-kneed liberal) by telling them that aborted babies will find their way to heaven. The two heresies began to work together and feed off each other.
The idea that unbaptized babies go to heaven is a heresy; it is part and parcel of the heresy of universal salvation.
If you ask virtually any Conciliar priest (go ahead, ask an FSSP priest or a "tradition-friendly" Novus Ordo priest) if aborted babies go to hell and I doubt that you will find many who will answer in the affirmative. There will be a few, but it will be precious few. Many will answer unequivically that they are saved. Others my equivocate, and their answers will be muddy; but, make no mistake, they will leave you with the impression that he told you (even if he did not actually say it) that these babies will go to heaven. And a very distinct minority will tell you that these babies will go to hell for they have not Sanctifying Grace.
It is clear that this heresy is the teaching of the Conciliar church and that the vast majority of people who worship with the Conciliar church believe in that heresy with all their heart, soul, mind, and strength. It is what they teach. Their preaching against abortion (that once-a-year sermon on "Respect Life Sunday") is based only upon humanistic reasons and not on spiritual reasons. While the human element should indeed be discussed, it is not the primary reason abortion is wrong and these reasons cannot be the sole reasons against the practice. The "Golden Rule" just doesn't work with people who have lost faith in the One True God and believe in the evolutionary model of "survival of the fittest."
So, to answer your question on the modern origin of this attitude amongst the people of the Novus Ordo, it is simply a loss of faith and a clergy who refuse to see or teach the Truth.