tumbles occur when

Tumbles occur when the flagella of a bacterium rotate clockwise. This causes the flagellar bundle to come apart, which disrupts the bacterium’s forward motion and triggers a random reorientation or erratic movement. During tumbling, the bacterium changes its direction randomly before a new run (forward swimming) begins. Tumbling is part of the run-and-tumble behavior seen in bacteria such as Escherichia coli, and it helps the bacterium navigate its environment by alternating between runs and reorientations. Tumbling allows the cell to randomly change direction to find more favorable conditions, especially in response to chemical attractants or repellents. Essentially, tumbling happens when the flagellar rotation switches from counterclockwise (which forms a bundle pushing forward) to clockwise rotation (which causes the bundle to unravel and the cell to tumble).