Easy to see you’ve never had a baby. 
The head is the big part of a baby, and in a normal delivery, the head comes out first. The uterine contractions, repeated strongly throughout the many hours of labor, have the effect of actually molding the baby’s head, whose skull bones are not fused together at the top, into a more oblong shape, to make it easier for the head (the tricky part) to pass through the opening in the mother’s pelvis. Then, once the big part is out, the rest of the body slips out quite fast (pops out).
However, in a breech birth (feet first), which is basically what you’re talking about here, you’ve got the skinny, small part coming out first while the head is still up inside the uterus, without allowing the hours and hours of uterine pressure on the upside-down baby’s head in the pelvic opening to mold the head into a more ergonomic design. And in this case there’s a very real risk, depending on the size of the baby and the size of the mother’s pelvis, that the baby’s head will get stuck on the “still inside Mom” side of the pelvic opening like a cork in a bottle, and the baby will just “hang” there–can’t go forward, can’t go back. Then you’re looking at an emergency c-section (and in earlier eras, the mother and baby both simply died.)
So a breech presentation baby can’t physically “pop out” while the doctor is trying to maneuver his sharp pokey thing up past it in the vagina. Its big, round, unmolded head is still inside the uterus and is still locking the baby in place inside the mother’s pelvis. Even granted that you’re talking about a fetus that has a smaller head than a full-term baby, still the laws of physics will hold the Big Part up inside the mother, so it can’t “pop out”. Visualize a Tootsie Roll Pop inside your mouth. Now visualize your pesky kid brother trying to yank it out of your mouth–he can’t, as long as you keep your mouth closed around it. The uterus is closed around the baby’s head the same way, at the cervix.
The accounts of births that have the baby “popping out” because it’s slippery are generally speaking accounts of normal, head-first births, in which the uterine contractions were allowed to do their job of squeezing the baby’s head into a more manageable shape. Or they are accounts of natural premature births, when the baby’s head wasn’t as big as it was supposed to be, so it only took the uterus a short period of contractions to push the whole thing out, since it was overall much smaller and easier for it to fit through the pelvic opening.
But a late-term abortion like you’re talking about doesn’t involve any uterine contractions, so there’s no way for the fetus to be pushed out past the doctor’s hand, even with a smaller head. And with the cervix holding the head in place, it can’t slip out, no matter how slippery the vagina may be.