I believe there are several reasons why bounty hunters sell their rewards immediately they get it. Some it may be due to how potential the token is and the level of trust they have in it, If it is a good one with good features, you can hold it till the price is higher and if not, they decide to sell. Some also sell if they have financial need that needs to be attended to urgently and some sell to buy another stable coin. The reason varies from individual to individual.