As a cop, he whistle blew on a cop who was framing black teens for crimes and had raped someone, which resulted in his police department to try to sideline him and eventually fired him.

Here is more articles on the matter

https://steemit.com/news/@johnvibes/ufc-fighter-turned-cop-fired-for-reporting-fellow-cop-for-rape https://www.boston.com/news/local-news/2016/11/30/boston-police-officer-mma-fighter-appeals-desk-duty-ruling/ https://patch.com/massachusetts/boston/sjc-sides-ex-mixed-martial-arts-fighter-boston-cop-against-boston-police https://www.reddit.com/r/IAmA/comments/5i95lp/im_ufc_vet_sean_gannon_the_only_man_to_ever/db6dn9s/