“Unsuitable” for HPD

My friend recently went through the extensive Houston Police Department applicant process, only to be turned down on grounds of “unsuitability.” He passed all his physical and psychological tests, doesn’t have a record, and has no major red flags from what I can see, other than a gun “hobby” (owns quite a few). He seemed to really expect he was getting in, so I’m wondering what “unsuitability” really means. Give me the low down, please.