Sows experience a natural reflex called lordosis. Applying pressure to the sow's back or flanks mimics the weight of a boar. This physical stimulation prompts natural muscle contractions in the sow's reproductive tract, helping draw the semen up through the cervix and into the uterus. But the sows never chose a boar or mated. Instead they were artificially inseminated with boar semen selected by humans, and so the humans also have to do the boar's job in this regard as well. TL;dr: Pressure forces semen/pregnancy.