Because of dumpers, bounty hunters, cause they want immediate profit after they already get thier shares. This scenario always happen when a coin is being listed in an exchange .
The dumpers are the reason behind most project fall in price listing on exchange but I still don't agree that the bounty hunter are the main cause of it. However, if the project is good people won't plan to dump such coin and if the coin was dumped. If the duty of the team to rectify it either by listing the project on CMC or other means.