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.