I think Magnus Huber has suggested this might be the result of Nigerians ending up in Sierra Leone because of British interception of ships leaving West Africa and then making their way home, having had quite a bit of contact with Krio. There were also Sierra Leonean missionaries active in Nigeria and Cameroon who might have had an influence. It's really interesting how many similarities there are between Nigerian/Cameroonian pidgin and Krio, given the geographical distance.