What could causes an abnormal pap smear?

Many causes. Inflammation, precancerous changes usually due to hpv changes, hormonal changes (esp postmenopause), smoking!, and infectoins.
Usually HPV. A vaginal infection can sometimes be the cause of an abnormal pap smear. A cervical infection can as well (chlamydia, gonorrhea). The most common cause would be infection with human papilloma virus (hpv).