I'd say just about all of these things are root causes in at least some cases:
Men: Man rapes woman, she gets pregnant and doesn't want to have rapist's baby.
Poverty: Woman does not feel economically capable of supporting child.
Lack of chastity: People are not chaste, have sex when they are not prepared to be parents, have abortions.
Low self-esteem: woman has sex out of need for love/validation, is not ready to be parent though, has abortion.
Contraception fails: duh.
Social Conditioning: Whatever you may think of it, abortion is seen as a viable and acceptable way to deal with unwanted pregnancy in our society.
Drug abuse: leads to situations of poor judgment/loss of inhibition or desparation, leading to sex...
Alcohol abuse: Leads to situations of poor judgment/loss of inhibition....
Promiscuity: People want/like sex who aren't prepared to be parents. Leads to unwanted pregnancy when combined with ignorance about birth control or other non-risky sex options, etc.
Immorality: I suppose one could argue that abortion is unethical, though the premises on which one would base such an assertion are certainly arguable.
Sexual exploitation of women: Woman in an unequal position of power with respect to a man feels unable to deny him sex or to make him use birth control, even though she would wish to.
Other: Could be any number of things.
Rape: See Men, above.
Naivety: Woman assumes man will want to have child and family too, or something like that.
Overall, though, I'd say that far and away the #1 cause of abortion is ignorance. Ignorance about birth control options, about how to use birth control most effectively even if it is used, about how to have sex in ways that can't possibly lead to pregnancy, etc.