What is the current accepted theory as to why Venus has a slow retrograde rotation?
According to this NASA overview, the planet Venus is unique (amongst the major planets), Venus has a slow retrograde axial rotation, taking 243 Earth days to make one rotation (which is longer than its orbital revolution).
What is the current accepted theory as to why (and how) Venus developed this anomalous slow retrograde axial rotation?
There seem to be a few, and none are accepted by the whole scientific community. The main ones:
- Venus was struck by a large body during its early formation
- The spin axis flipped, as can happen with a gyroscope
- The spin slowed to a standstill and then reversed, caused by the sun's gravity, the dense atmosphere and friction between core and mantle
That final one seems to be the most recent, being proposed by Alexandre Correira and Jacques Laskar in 2001. Their research seems to imply that the conditions on Venus and its distance for the sun make a retrograde spin slightly more likely than a forward one.
To a first order approximation, the third explanation has to be wrong, because (as of 2012) Venus is known to be slowing down. See Could Venus be shifting gear?. If it slowed to a standstill and then reversed, for any gravitational reason, then to first order it would now be speeding up. Of course, there might be a second order oscillation, but that's more complicated and, I assume, not what the authors (back in 2001) were proposing.