Over-kneading sourdough dough results in a tough, dense bread with a less open crumb structure due to gluten breakdown. The dough becomes slack and difficult to shape.
Kneading develops the gluten in the dough, creating a network that traps gases produced during fermentation. This network gives the bread its structure and elasticity. However, over-kneading weakens this gluten network. When sourdough is over-kneaded, the long gluten strands break down, resulting in a dough that is slack, sticky, and difficult to handle. The final bread will be dense and tough because the gluten structure is no longer strong enough to support the rise and create an airy crumb. The dough will also lose its ability to hold its shape during proofing and baking, leading to a flat, dense loaf.
Instead of relying solely on kneading time, pay attention to the dough's texture. Sourdough is usually ready when it's smooth, elastic, and slightly tacky. The windowpane test (stretching a small piece of dough thin enough to see light through it without tearing) is a good indicator of proper gluten development.