Terms and Conditions
There are various hypothesis for this
You must know that earth rotates at nearly 23 degree, same way Venus rotates at 177 degree
1. The first and oldest hypothesis was that large meteoroid must have hit the Venus and lead to rotate backward, but if it was that large enough to change its rotation, meteoroid must have destroyed the planet also, but this was not the case.
2. The second and the latest hypothesis is that the planet slows down when it lost its own force and starts rotating backward due to Sun's magnetic pull. This is somehow scientifically evidential.
Thank you for asking such an interesting question.