When roosters mount hens, they hold on to the feathers on the back of their head and literally stand on their backs/wings. If a roo is doing this particularly often to a hen, it'll result in broken and missing feathers in those areas. It's normal because roos tend to play favorites even in large flocks (not so normal if he only has a few hens, though), but if they get too bald there is the risk of skin damage, so you may want to look into a chicken saddle and consider getting some Blue Kote for exposed skin on the head.
Now if you see a roo just plucking feathers from a hen for no apparent reason, then he should be culled.