That is usually caused by lifting with a chain that wraps over the top around to the bottom cutting edge. Then the operator lifts something too big and heavy and stalls the loader. Then the operator rolls the bucket back, lifting the too heavy load with the roll action of the bucket. Massive force is applied to the topedge of the bucket from the roll and lift action.
This is particularly effective at bending the bucket if the cutting edge is butted up against the object at the ground level, or if the operator has it in reverse yanking at the same time.
That looks like a Ford 4000 series tractor (maybe a 3930?) with the quick detach loader frame, carrying a material bucket? I'm an expert at bending those.