Even Wikipedia (not exactly known for its honesty on controversial matters) admits that this was the motivation:
"In 1959, Dr. Bent Boving suggested that the word "conception" should be associated with the process of implantation instead of fertilization. Some thought was given to possible societal consequences, as evidenced by Boving's statement that "the social advantage of being considered to prevent conception rather than to destroy an established pregnancy could depend on something so simple as a prudent habit of speech." In 1965, the American College of Obstetricians and Gynecologists (ACOG) adopted Boving’s definition: "conception is the implantation of a fertilized ovum.""
In short, it was considered "prudent" to tell women that the birth control pill only prevents pregnancy and not that it can end the life of a child. In order to do that, they defined pregnancy to begin at implantation so as to hide the fact that human children who had not yet been implanted could be killed by this new "contraceptive." If pregnancy has not yet begun because the baby has not implanted, then there was no abortion. Thus, preventing implantation is not counted as abortion on a technicality, due to this faulty definition of pregnancy, despite the fact that it does kill a human individual.
In fairness, this is not the main mode of action. There are three mechanisms by which hormonal birth control works. The main mechanism is to prevent ovulation. If no egg is released, there can be no conception. This mechanism is truly contraceptive by preventing conception. The secondary mechanism is to thicken cervical secretion to impede sperm motility. If sperm do not reach the egg, then no conception occurs. However, the third mechanism works as a backup in case the first two mechanisms fail (which they sometimes do). The uterine lining is thinned by the hormones and becomes less able to care for an implanting child. A thin lining means less blood flow and difficulty in connecting to a blood vessel. If an egg is released and fertilized, then the new child will usually fail to implant in the womb. The baby then starves and dies. There is no way of knowing which is happening each month for any particular woman.
Here are some secular health sites that discuss the mechanisms for hormonal birth control. It is well-known that the uterine lining is affected and may prevent implantation. In fact, one of the main beneficial effects they tout for birth control pills is that they thin the uterine lining and produce lighter periods or even none at all in some women. A thinner uterine lining means a baby cannot implant and get the nutrients he needs.
"The hormones in the Pill can also sometimes affect the lining of the uterus, making it difficult for an egg to attach to the wall of the uterus."
"Birth control pills contain synthetic hormones that work to prevent pregnancy. These hormones can stop ovulation and make it more difficult for sperm to enter your uterus. They can also alter your uterine lining, which can reduce the likelihood of implantation."
"The pill also thickens cervical mucus so the sperm cannot reach the egg. It makes the lining of the uterus unreceptive to the implantation of a fertilized egg."
The abortifacient potential of the combination birth control pill is shared by all hormonal contraceptives because they all contain the same progestin hormone that thins the uterine lining. This includes the regular birth control pill, the mini pill, the implant, the ring, the patch, the depo shot, and hormonal IUDs. All of these have the same mechanisms. They all contain progestin. They all thin the uterine lining. They can all prevent a human embryo from implanting in the womb.
The copper in the copper IUD does kill sperm as the primary mechanism, but it also inflames the lining of the uterus and makes it inhospitable to an embryo, if fertilization were to occur. It does appear that preventing fertilization occurs more often, but killing an embryo can occur on occasion. Again, we don't know how often each mechanism is occurring in any particular woman.
"All IUDs induce a local inflammatory reaction that disturbs the functioning of the endometrium and myometrium and changes the microenvironment of the uterine cavity. Moreover, these effects alter signaling between uterus and ovary. The entire genital tract seems affected, at least in part because of luminal transmission of fluids accumulating in the uterine lumen. Copper or progesterone-releasing IUDs may attenuate or accentuate the inflammatory response, disturb the physiology of the gametes in the female genital tract, or destroy the viability of the embryos or endometrial receptivity to implantation." (Emphasis added)
"All intrauterine devices (IUDs) that have been tested experimentally or clinically induce a local inflammatory reaction of the endometrium whose cellular and humoral components are expressed in the tissue and the fluid filling the uterine cavity. ... In the human, the entire genital tract appears affected due to luminal transmission of the noxa that accumulates in the uterine lumen. This affects the function and viability of gametes, decreasing the rate of fertilization and lowering the chances of survival of any embryo that may be formed, before it reaches the uterus." (Emphasis added)
Note that you have to read the fine print carefully on this issue because a lot of resources will tell you that IUDs are not abortifacient because the primary mechanism is to prevent fertilization. While it's true that the primary mechanism prevents fertilization, an occasional abortion is ignored, yet is very much an important moral issue that should be considered. They skip over that part on purpose in order to persuade women to use such devices. If you look at the scientific literature, they do admit that IUDs, including the copper one, can kill an embryo if fertilization does occur.
There is one more objection that some will use to argue for these birth control methods. It goes like this. A large fraction - estimated from 10-40% - of human embryos will die naturally even without these birth control methods. In fact, they will even claim that since these methods prevent so many fertilization events, the total number of babies dying goes down. Yet this does not justify occasionally killing human babies.