 Why did the robot at the stream fall into the mud?
Frowning, worrying about the sounds, Alan momentarily forgot to watch his step until his foot suddenly plunged into an ant hill, throwing him to the jungle floor. Without pausing to think, Alan leaped back, and fell sprawling over a bush just as one of the robots rolled silently up from the right, lowering its blaster barrel to aim directly at his head. Behind, the robot crashed imperturbably after him, lighting the night with fitful blaster flashes as some winged or legged life came within its range. Alan, lying in the mud of the stream bed, felt the earth shake as the heavy little robot rolled slowly and inexorably towards him.
 It fell while trying to chase Alan, who managed to confuse it.
 Alan managed to knock it down with his blaster.  It fell while trying to chase Alan, who managed to confuse it.  The mud blocked its sensors and it did not know how to move properly.  Its signals were disrupted and it malfunctioned.