What causes some women to have repeated abortions?
It is certain that a desire to have sex without contraception and a lack of desire to be a parent cause women to have multiple abortions. (precluding rape)  


A certain amount of a desire to have sex is natural. Causes for increased sexuality are so numerous that is not tenable to search.
Risk taking behaviour is associated with sex without contraceptions. A stable long term relationship might be one reason to have sex without protection where the risk is perceived to be less based on the lack of infidelity. A lack of responsibility could also motivate someone to consistently not use protection. Peer pressure by the male or even religion is also possible. 
Finally there are many reasons people decide not to be a parent. Primarily they are unable to be a mentor and they are incapable of nurturing. There are many additional reasons at this link about why certain reasons to have children are irrational. However it should be stated that just because one is incapable of being a good parent doesn't leave abortion as the only option. The child can be given up for adoption however that is not popular.