Norwich Crown Court heard PS and JE Ward Ltd had breached three sections of health and safety laws.
The company had earlier been found not guilty of corporate manslaughter but Judge Stephen Holt said "nevertheless a crime had been committed".
"No-one knows why Mr Pieton ventured into the area he did and he was an experienced tractor driver," he said.
The court case had established the responsibility of the company for its employees under the Health and Safety Act, he said.
Grzegorz Pieton may have acted on his own initiative but this did not diminish its responsibility, the judge added.
He imposed a fine of £50,000 with costs of £47,932.