Who attacks the steamboat as it approaches the Inner Station? Why?

As Marlow's steamship approaches the inner station in a heavy fog, arrows begin to fly out from the jungle. The "pilgrims," or white passengers, fire blindly into the jungle with their rifles. Marlow blows the steam whistle on the ship and scares off the attackers. Then Marlow realizes the helmsman has been killed. In the next chapter the Russian explains that the Africans attacked the ship because they were afraid the ship was coming to take Kurtz away from them. Later the Russian divulges that Kurtz ordered the attack himself because he did not want to be taken away from his base of power.