What causes the Earth to rotate on its axis? Why is the axis tilted?
It has to do with the way the solar system was formed. When all the matter clumped together to form the earth, it clumped on that started a rotation. Either that or some large piece of material hit the earth with a glancing blow in its early years and started the rotation. A large impact such as this is also what created the moon.
The reason the axis is tilted has only to do with the start of the rotation. Once a rotation is started, especially with a mass the size of the earth, its very hard to stop the rotation or change the tilt of the axis.