Doesn't surprises me much. In many cases, the bounty rewards are delayed by months. The participants may get their tokens many months after the bounty program actually ends. And if the participant is short of funds, then he will not think twice before off-loading the entire bounty reward in some exchange.
For me its a case to case basis. It depends on how an individual set their goals,for some who like instant cash the sell it right away. Other reason is they are worried when market go down and afraid they lost so much if they hold it for a long time that's why they give up and sell it.