From my experience as a top, it's a combination of the physical act of my cock spasming and pulsing inside a bottom mixed with the psychological effect of knowing I'm cumming and in ecstasy inside of them that seems to drive my bottoms crazy and have an anal orgasm around my cock. Not only does the skin to skin contact make feeling my cock cumming that much more possible, but I think the anticipation of me cumming in them really drives my bottoms crazy. It's like with every thrust I'm getting closer and closer to shooting my load in them, and just knowing that prepares them to go over the edge and their asses to orgasm when I cum in them. SO damn hot!