This is quite a possibility but if you look at what percentage of the total tokens or coins that bounty hunters are holding at any given time then you will realize that the amount can be insignificant. I am more inclined to believe that once a token/coin is already in an exchange, big investors who owned big amount of the asset are cashing out all because they understand that eventually the value is going down...and lastly the owners and developers of the projects are actually unloading their shares so they can make some profits too. I don't think blaming everything to the bounty hunters why there is a big dump in value can be fair enough as most projects are just allocating 1-3% of the total supply available for bounty promotions (and many are not even paying or increasing all tokens to the bounty especially where there is a KYC required).