A KYC following big wins is a common practice shared by many [if not all] casinos, big or small, but mostly big, old or new, but mostly old. As small and new casinos will try to make themselves as lenient as possible to gain traction, while older and bigger casinos are more concerned with securities and betting integrity.

Why now and not when he was depositing or losing, well, technically he can do that. BC's KYC is open for their players to submit and verify anytime they want.