Why do some planets have reverse rotation?
Almost all celestial rotation in our solar system is from West to East, or counterclockwise when looking down from the North pole. All planets orbit the Sun in this direction; the Sun itself, as well as all but two planets rotate in this way. This is not too surprising, since the angular momentum associated with all this rotation is the same as the total angular momentum of the primordial solar nebula from which the sun and all planets formed.
The two exceptions are Venus and Uranus. Uranus' rotational axis is tilted by slightly more than 90 degrees with respect to its orbital plane, i.e. it is rotating “on its side”. Venus is the only planet in the solar system that truly exhibits reverse rotation, spinning in the opposite direction to its orbital motion.
It is thought that this anomalous rotation was caused by a collision with a very large asteroid, or even a planetoid, early on in the history of our solar system. The collision would have destroyed the asteroid, but the impact could have changed the orientation of the orbital rotation axis.