The erratic movement of the flexible pipe during the shower is attributed to a phenomenon known as firehose instability. This occurs when the water pressure causes the pipe to whip around uncontrollably, resembling a snake in motion. The instability is a result of the dynamics of fluid flow and pressure differentials within the pipe. Understanding this behavior can help in designing more stable plumbing systems. Overall, firehose instability illustrates the complex interactions between fluid dynamics and flexible materials.