It is the tightness, and probably the slightly dry sensation. It generates more feelings to the nerve ends of the head of the penis, those tiny little bumps. Whereas by the time you get to anal, often the pussy is so wet that it can have the effect of softening the penis. I have withdrawn for a rest and because my partner was gushing, just to keep it hard have rubbed along the perineum but because it was so wet slipped into the butthole by default, even without realising it.