Grouwels was riding Sanchez in a 1m class on Sunday at the unaffiliated event, which is run under affiliated rules.
"Mr Grouwels' horse refused at a simple fence - the first part of a combination - the rider insisted and the horse somersaulted over the fence," FEI safety office for Belgium Tony McPherson said.
Grouwels was killed instantly in the fall.
He was an experienced rider and had been captain of the Geetbets riding club for three years.
Club member Corry Lambrechts told the Zutendaal newspaper that riding was Grouwels' life. "We have one consolation: Dirk died when he did what he enjoyed doing. He himself had three horses, including Sanchez, who has been restless since the accident."
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