Usually they only allow an applicant to apply to two divisions within their firm. Generally a non-target will apply for IB and the Back office role that the company recruits at their campus for.
We all know applying online from a non-target is really pointless. Now, you are stuck with your Back office role as your last option. However, they see you applied to IB and back office - both completely different jobs. They realize you are only using the back office spot as your Plan B. They recognize you aren't really interested in that, so they do not even bother interviewing you. Now you are stuck with nothing.