A platoon of soldiers marches on a road in steps according to the sound of a marching band. The band is stopped and the soldiers are ordered to break the steps while crossing a bridge. Why?

When soldiers are marching in steps, their feet comes on the ground simultaneously. This increases the magnitude of the force on the bridge which may break it. When they break the steps, the force doesn’t act on bridge simultaneously and the force oscillations are not produced to break the bridge.