Hello, PKing administrator here,
The O'Haras decided to send a order out to kill you for suspicion of becoming a federal agent. Regarding the PK reason, the ruling on these is pretty loose and you can die for any valid reasoning your HC gives. This was made aware to you upon joining the O'Haras via the popup when whitelisted. The reason this was valid was because in the PK rules and guidelines, they can order to kill you if they have reasonable suspicion that you are a rat, whether it is actually true or not.