Why is my one guppy chasing and biting the other guppies?

i have a 10 g. tank, i have 3 male red guppies, and an african dwarf frog in it. the ADF just sits at the bottom. they have been together for about a week now, with no problems, and now out of no where one of the guppies is chasing the two others around, is theres something i can do? add salt or something? or is it normal?