How is it possible?

I played at party.I had KK and raised and lead against TT. T fell at river and I bet and got raised and called. I muck losing hand, So I think nobody could see my card.However Some at the table said bad beat, the opponent said maybe he has been behind all the way, So obviously he did not see my card. Some said KK ,exactly what I had,How could he know what I had?