Question: Given: Law of Conservation of Angular Momentum.

* Reverse spinning with dense atmosphere (92 times > Earth & CO2 dominant sulphur based).
* Surface same degree of aging all over.
* Hypothetical large impact is not a sufficient answer.

Assuming any object large enough to alter a planets rotation or even orbit would likely destroy most of its shape, yet Venus has retained a spherical property with a seemingly flat, even terrain indicating no volcanoes,and few if any visible meteor impacts. It would be fragmented and dispersed for billions of years. Even the question of what meteor, comet, asteroid composition could survive traveling that close to the sun's temperature, radiation, electromagnetic energy, solar flares, or gravity to equal a mass reactionary change as to alter it's spin.

Answer: 
IMO there is no solid explanation, as anna said. [Only clues (WP)](http://en.wikipedia.org/wiki/Venus#Orbit_and_rotation).

In this simulation (2002) [Long term evolution of the spin of Venus- II, Numerical simulations](http://www.imcce.fr/Equipes/ASD/preprints/prep.2002/venus2.2002.pdf) we find a mix of: 'chaotic zone', instability, large impact, close encounter, tidal effects, planetary perturbations,... 

**There is room for speculation:**  

I think that the heavy atmosphere is not a significant factor of slowing the rotation.  

I discarded any violent event because it can easily change more than one parameter; in this case we have only one (the direction).  

The planet's minute axial tilt (less than three degrees, compared with 23 degrees for Earth) make me think to keep only the **tidal perturbation**, although the present configuration Sun-Venus-Earth is not able to justify it.  

As the Venusian surface rotates as low as 6.5 km/h (on Earth is about 1,670 km/h) we can think that Venus may have changed the direction of rotation not long time ago.  

The solar system angular momentum problem is not solved (the planets strangely have almost all of it) and I think that the solution of this does one is not related to the present question.  

The **equation 35** of this paper ([a new model, undiscussed](http://arxiv.org/PS_cache/astro-ph/pdf/0208/0208365v1.pdf)) allows the slow evolution of the configuration Sun-Venus-Earth.
Well, I Binged and found some [references.](http://www.astronomycafe.net/qadir/q50.html)

Seems that a collision is most probable, if it happened at a time when the whole system was malleable. But there is no solid explanation.