Shockingly, most do.
The pill works by suppressing ovulation, however, if ovulation does occur, and the ovum is fertilized, the newly conceived baby will find a hostile environment in the uterus, because the pill has a back-up plan just in case. It causes the uterine lining to not support the implantation of a conceived zygote (very early stage of a baby).
The mini-pill does not prevent ovulation, its only contraceptive mechanism is to make the uterus inhospitable to implantation.
The injection (Depo-Provera) works in the same way.
Intra-Uterine Contraceptive Devices also abort a newly conceived pregnancy.
The morning-after pill causes the new life to abort.
Barrier methods, such as condoms and diaphragms prevent conception from occurring, therefore they do not cause an abortion.
Timing methods such as Natural Family Planning do not cause abortion, they simply avoid the fertile time when conception is most likely.