Answer:
He was found out to be a fake!
Ciaran O'Keeffe, the shows Parapsychologist, had suspicions about Derek being a fake. None of the other members of the crew realised this though. So on one of the investigations Ciaran mentioned to a member of the crew that someone called Kreed Kafer (Ciaran had made up this name) was haunting that particular building. He made sure he said it within earshot of Derek.
Then I think Derek got possesed by someone he called 'Kreed Kafer'. This is how they knew he was fake. Kreed Kafer is a completely made up name and turns out to be an anogram of 'Derek Faker'.