Its because lots of bounty hunters needs money immediately thats why they sell their tokens that they received in the bounty when its already in the exchanger. And i think other reason is the value of token came from the fresh ICO is cheap at first release and they never assure if it will have a high value.
Maybe because they know that they can get a higher earning so they sell it immediately or they see the market situation that the market is having a pumping so they immediately sell it to get a huge profit or they really need money so they sell it. But i think its better to hold but if you know the market is rising then sell it so you can get higher profit, but if not then hold for a while until market rise rapidly.