The pilot reported that during the last long-line load of the day moving bags of cedar mulch, while on approach to the drop zone, he looked outside at the trees, and did not see any indication of wind; however, he felt a little shudder as the helicopter came out of translational lift. He again looked outside at the trees, and did not see any indication of wind, and continued his approach to the drop zone. Once over the drop zone, as he went to release the bag, he noticed that the remote hook plug had come unplugged. He decided to pick the load back up and move the bag over to the pile of mulch so he could land and manually unhook the bag. When he picked the bag off the ground, the helicopter started to spin to the right, which he was unable to stop. He noticed that the collective was in the full up position so he lowered it to stop the spinning; however, before he could arrest the spinning, the helicopter struck a pile of mulch, and rolled onto its right side, which resulted in substantial damage to the fuselage. The pilot reported no preimpact mechanical malfunctions or failures that would have precluded normal operation.