An employee was standing behind a trash truck after bending over to pick up a piece of debris. City employees were emptying a roll-cart into the back of the trash truck. When the empty roll-cart was lowered back to the ground, it struck the ground with enough force to dislodge from the mount on the back of the trash truck and bounce away from the truck, striking the employee. The employee was knocked to the ground, resulting in a concussion and a head laceration.