That could be the result of sterility from sexually transmitted diseases and/or the spread of homosexuality.
The original articles cites no explanation:
“Meanwhile, U.S. abortions have been dropping to their lowest levels in decades, according to other reports. Some have attributed the abortion decline to better use of contraceptives, but other experts have wondered if the rise in births might indicate a failure in proper use of contraceptives. Some earlier studies have shown declining availability of abortions. Cultural attitudes may be a more likely explanation.”
How would homosexuality result in fewer abortions, though? Theoretically, there would be fewer births, not fewer abortions.