I grew up on a sheep farm, herding them on foot and on a bike. There was clearly never a leader. It always made the most sense in my head to treat them as a bubble of hyrdoscopic liquid with yourself blowing at them through a straw. You pushed around the edge to get them to go in the direction that you wanted, but had to be careful not to push too hard when they got to a choke point, or they'd splatter everywhere. Different factors, such as the presence of rams or lambs would increase the reaction, causing them to be more or less likely to break up and more reactive.