There is no motive. Any other month in 2013 would have made more sense. He would be insanely crazy to dump coins at such a low price when it was 30k satoshis only a few weeks earlier.
There's plenty of motives. Maybe he thought the coin was losing value and this was his chance. Maybe he'd been thinking about it for a while but didn't have a plausible cover story.
Saying he didn't have motive is a very weak defence.
Again, conjecture, but there are 101 reasons he might have decided to dump the coins.