The pilot of the helicopter reported that he noticed a slight bounce during flight that felt like an out of balance main rotor blade. The pilot diverted to a nearby airport as the helicopter began to shake aggressively with a whipping noise heard. During the approach, the shaking worsened, and the pilot elected to perform a run-on landing at the airport. During the following inspection, it was revealed that the main rotor blades were substantially damaged due to delamination. There was also evidence of impact marks near the tip blades that was consistent with contact with vegetation. The operator reported that, 2 days before the accident flight, the blades may have impacted vegetation during an “off-site” landing. According to that pilot, while the front left seat passenger was exiting the helicopter, she inadvertently sat down on the left side collective, pushing it to the full-down position. The helicopter rolled backwards suddenly, resulting in the heels of the skids contacting the ground. The pilot corrected the helicopter’s attitude and checked that the collective’s movement was normal. The helicopter was shut down shortly after and tail and main rotor blades were inspected by the pilot who reported that he didn’t see any evidence of impact at that time. Maintenance personnel inspected the helicopter following the flight and no statements or writeup was found during the investigation. The three pilots that flew the helicopter after the unidentified impact all reported that they felt a slight bounce in the helicopter at speeds of approximately 115 knots and thought that it was a slight out of balance/track rotor and was not that significant.