Stewart ran into Kevin McDowell, the right rear tire changer on the No. 16 Ford, during a late pit stop. Television replays showed McDowell slump to the concrete in pain after Stewart hit him leaving his pit stall.
There was no immediate word on McDowell's injuries or condition.
Biffle finished 32nd in the Coca-Cola 600 and dropped two spots, to ninth, in the points standings. Stewart was 15th and fell two positions to 16th in the points.