That is normal in a male only tank. Male guppies have an irresistible compulsion to breed. When no female is present they choose a male, usually the largest one, and court it. They choose the bigger one because females are usually larger than males. I have seen males try to court neon tetras and mollies. As long as the picked on one has a place to hide and rest he will be fine. Large clusters of plants, real or fake, work well.