It is very tough to observe the rotation of Venus due to its featureless thick atmosphere makes this impossible to observe even through telescope. Venus rotates in clock wise direction. It was discovered in 1960 that its rotation is 243.09+/- 0.18 days long. It rotates in a backward direction in retrograde form.
For years there was a misconception that this backward rotation of Venus is due to Earth but it is not so it rotates on its axis of 729.27 days the earth goes twice round the sun. On the other hand Earth rotates round the sun in 728.50 days. In 3:2 ratio Earth and Venus are locked in a tidal resonance.
Forces which spin they spin in particular acrobatics. This happens because of moons tidal attraction between the planets. In case of Venus the retrograde is in opposite direction due to the influence of 3:2 resonance with Earth.
Some of the smaller planets which rotates around the sun might have collided with Venus which increases the weight of Venus in ancient times and which in turn increases the weight of the Venus and hence the rotation of the planet gets affected for that.