According to this special report from the Araphaoe County Sheriff's Office, Steinmier, 73, "had been trying to start the airplane and discovered that the battery was dead. He left the ignition on, got out of the plane and attempted to start it by manually turning the propeller."
Bad idea, since "the engine started and the plane began to taxi down the runway unattended."
According to the report, "Mr. Steinmier unsuccessfully attempted to climb back into the cockpit," after which "the plane continued on for several hundred feet, crossing over a taxi area and a runway, damaging some airport runway signs. The aircraft then hit the grassy area on the far side of the runway, bounced and flipped onto its back."
The 1978 Cessna 182 Skylane II in question "was valued at about $80,000 and is a considered a total loss," the ACSO declares. Although, if anyone has footage of Steinmier chasing after his plane, he might win a prize on America's Funniest Home Videos.