The pilot reported that during the landing roll, at a point about two-thirds the way down the runway, the airplane veered to the left. The airplane continued off the left side of the runway, through the rough terrain, over a bump, and into a ditch. The left wing sustained substantial damage. The pilot reported that there was no mechanical failure/malfunction of the airplane, and that due to the tight foot space; he most likely inadvertently applied the left brake which resulted in the airplane veering to the left.