As a tandem paddlecraft's speed increases, it's functional rotational center, the loci of all forces on the hull moves forward to just behind the bow station. A bow paddler, who's stroke averages maybe about two feet from that rotational center.,\ Failure to use a vertical shaft or stroking along the rail will result is offside sweeping forces turning the bow towards the stern's side, but the moment is small, and the bow's aft blade carry so limited by the hull that the bow cannot induce yaw without doing something obvious and heroic, like a draw or cross draw.
The stern, on the other hand, has an eight to ten foot moment arm from stroke center to the Loci, so it doesn't take much to torque the stern off course. The sterns sins are often multiple; failure to employ a vertical paddleshaft, stroking along the rail not parallel to the keel and carrying the blade past his body. Any or all three result in sweeping forces which turn the canoe onside towards the bow's paddle side. The J corrects for all those inadvertent sins, but it's better not to torque the hull of course in the first place.