I think because they just want to tkae profit as fast as possible. Most bounty hunters are not financially sound and so as soon as they receive their tokens and it is listed on any exchange sites they sell most of their tokens as soon as possible.And they want to enjoy the reward first like me.
maybe more than 50% of the bounty hunters are like what you have said, many bounty hunters are not financially free so when they receive tokens and it is already listed they sell it for them to comply for their daily needs, but also there are lots of bounty hunters who just hold their coins because they see the future in holding.