Why do some planets spin backwards?

Most planets in our Solar System spin in the same direction they orbit the Sun, a leftover from the spinning disk of gas and dust that formed them. Venus and Uranus are the odd ones out: Venus rotates very slowly in the opposite direction, and Uranus is tipped almost completely on its side, spinning at about 98 degrees. Scientists think Venus reverse spin may have resulted from a combination of a massive early collision and the strong gravitational tug of the Sun on its thick atmosphere over billions of years. Uranus dramatic tilt is usually blamed on one or more giant impacts with Earth-sized bodies in the early Solar System. These events show how violent the young Solar System was as planets formed and settled.