men just like women often like to be teased regardless of sexual preferences so it could be just that.
but sometimes things come about naturally maybe he didnt even know he had a tendency for being topped. but if he's always denied it he might soon start to back track again and say it was a one off, denuseri's right you need to discuss it.
as well as many Dom/mes starting out as subs often subs start off as Dom/me or switches ..what happens if the balance starts to shift and suddenly you're topping full time, are you going to be happy with that?