Cannibals are often knocked down if hit enough with a melee weapon. When knocked down, the cannibal will be sitting on the ground, scrambling backwards with both arms and legs. This is presumably to avoid being hit by the player and to be able to stand again without interference from the player. Cannibals that are knocked down cannot attack the player. If left alone for more than a second or two, the cannibal will be on its feet again and attacking the player.
This mechanic is not to be confused with a cannibal being incapacitated.
Cannibal Behavior
Tribal cannibals, if not directly engaged in combat, will rush in to a knocked down comrade, pulling the knocked down cannibal to their feet. Starving cannibals may or may not attempt to eat knocked down cannibals like they do with incapacitated cannibals (needs testing as of v0.53c).