The AS-350B2 helicopter was refueled, and the pilot departed on a repositioning flight. Shortly after takeoff, the pilot noticed that the key for the fuel tank cap was not in the cockpit, and he elected to make a precautionary landing to a soccer field, to check the fuel cap. During the landing approach, the helicopter began to settle, it subsequently landed hard, bounced, and rolled over. The pilot did not report any mechanical malfunctions and stated he thought the helicopter entered into a settling with power condition. Calm winds were reported at an airport located about 4 miles east of the accident site, about the time of the accident; however, the pilot reported he experienced tailwind gusts during the approach.