Well obviously its because a lot of bounty hunters are immediately dumping/selling their coins massively after receiving their rewards from their respective campaigns. This can really bring a coin/tokens price down because mostly would accept any price lower as long as their coins/tokens are sold.
The fact that bounty hunters sell the coins they received is nothing wrong, and the price falls as there is no secondary demand for these coins after the end of the ICO, there is no product, there is no news about development, and the like.