Update (12:29 a.m.):
The Sporting News's Bob Pockrass has some more details:
Rich Willis, who was at the track, said he didn't see exactly what happened but his sister down in Turn 1 did. He saw Stewart and Kevin Ward get into the wreck.
"People (who could see it better) said the guy got out of his car and was gesturing angrily at Tony Stewart when Tony Stewart came by during the next lap under yellow," Willis said in a phone interview. "He approached him and evidently when he was driving by the guy standing on the track gesturing at him, he gunned his engine.
"What happened was the back end kicked out and clipped the guy and the guy flew across the track."
Update (1:06 a.m.): More from Pockrass:
Tyler Graves, a sprint-car racer and friend of Ward's, told Sporting News in a phone interview that he was sitting in the Turn 1 grandstands and saw everything that happened.
"Tony pinched him into the frontstretch wall, a racing thing," Graves said. "The right rear tire went down, he spun on the exit of (Turn) 2. They threw the caution and everything was toned down. Kevin got out of his car. … He was throwing his arms up all over the place at Tony for most of the corner.
"I know Tony could see him. I know how you can see out of these cars. When Tony got close to him, he hit the throttle. When you hit a throttle on a sprint car, the car sets sideways.
It set sideways, the right rear tire hit Kevin, Kevin was sucked underneath and was stuck under it for a second or two and then it threw him about 50 yards."