I have seen that happen before with individual rams.
When in a flock, the sheep (even any rams that may be in the flock) really do behave like sheep, and will be "frightened" of the sheepdog, hence enabling the dog, after suitable training, to round them up and move them in the direction the shepherd tells it to. The shepherd uses different calls, whistles and hand signals (which often made more obvious to the sheepdog by the shepherd waving his crook, or long stick) to tell the dog what it is supposed to do.
But a ram, by itself, doesn't follow the flock instinct (as it's not currently part of a flock) and, being usually quite a good bit bigger than the sheepdog, will sometimes stand its ground, and may even attack it with its forehead, as seen in the video.
For this reason, when a shepherd is training a young dog, he (or she) will try to make sure that it is not put in a situation where it would be confronted by a lone ram. Not just because the young dog may be physically injured when the ram charges it, but also because the mental fright to the dog may make it unwilling to face sheep in the future.