The thing is that the drill does make motion, and Autor does adjust his position slightly-- just slightly--to keep his balance. And the floor of the cave is the floor of the cave, and the least bit uneven, and fluid dynamics are dynamic--

Joly sees the bucket beginning to shift and steps forward to try to steady the ladder, or give a warning, or something. And then he sees the bucket finish shifting, as it tips over. He tries to dodge the water, slips, and only manages to get right under the now-empty bucket in time for it to land on his head with a hollow THOONK noise.