To make the Earth spin in the opposite direction may be several factors.

To make the Earth spin in the opposite direction can either impact with another celestial body of planetary size, or the influence of the star held by the Sun, the massive planets or captured by the Earth satellite, although this is debatable, writes

If you consider the collision, it is not easy will destroy all life on the planet, and dramatically reshape its entire surface, the planet will once again become a lump of magma.

If the rotation will be affected by the gravity of another body, without collision, then the process of changing the direction of rotation will be slow. It will cause strong seismic activity within the planet, as well as changes in the magnetic field, since the kernel will first stop and then spin in the other direction, which will lead to a complete change in the directions and intensities of currents of matter in the mantle.

The faster the turn the greater the seismic activity, which may make the planet unsuitable for life, on the other hand, if it is too slow, the Earth for a long time (thousands or millions of years) will remain without a magnetic field and the solar wind will burn and blow away everything from the surface. The Golden mean here, in any case, all living things die.