The native attack comes suddenly, without warning, and the first arrows strike the sounding-man, since he is at the prow of the steamer. "Sounding" is feeling in the channel with a long pole to make sure there is enough clearance for the steamer's draft, and so the sounding-man is the first person to appear in view.
"I was looking down at the sounding-pole, and feeling much annoyed to see at each try a little more of it stick out of that river, when I saw my poleman give up on the business suddenly, and stretch himself flat on the deck, without even taking the trouble to haul his pole in."
(Conrad, Heart of Darkness, eNotes eText)
Marlow is worried about the draft, and already on edge, and in the darkness, it appears that the sounding-man simply stops working and lies down, as if for a nap. Marlow is concerned, and then sees the fireman fall as well, and realizes that the steamer is under attack by arrows, almost invisible in the dark. Thematically speaking, the abruptness of the attack and the immediate death of a character gives the story more suspense. In-story, Marlow is spurred to take on the position of spotter, to keep the steamer from hanging up on snags or running into the banks, and so his personal safety is in danger.