Are there any truck drivers here? (Question regarding freight dispatch apps)

We have a driver who frequently transports goods for our company. Up until now, we have mainly used this driver, and when he was unavoidably busy, he would arrange for other trucks using freight dispatch apps (Hwamul-call). However, when we mentioned that we would call the dispatch apps ourselves, he strongly objected, insisting that we go through him.
Is it possible that he receives a commission per shipment when he uses the app to arrange other trucks? Is that why he is opposing this?