People who report incidents will often turn to the person or agency that they felt was most responsive to their needs. One can argue as to whether "responsive" means that the agency/officer was the most easily manipulated, or they were the one the reporting party felt the most comfortable with.
I cannot speak to any bias by the investigating detective, and chances are you cannot, either. The detective at the university could simply have been forwarding a report they received on to the DA and the matter was then dropped. That happens with some frequency in cases that involve domestic violence or accusations potentially related to it (like stalking, harassment, TRO violations, etc.). As long as the detective was still functioning within the law and agency policy, chances are nothing that occurred was untoward. You are certainly free to contact the UM police and make further inquiry. But, if you haven't been charged and your ex is simply screaming into the wind, it might be best to just let sleeping dogs lie.