They are likely attempted to mate them, male guppies are sex pests... as
@GaryE mentioned!
They, and platies, will also show off and chase each other when trying to establish a pecking order, so if you've added some of the fish recently, it's likely to settle down naturally.
Things you can do to help include adding decor and plants (live or fake, although live are better) that add some hiding places for fish to escape to when being chased, so they can get a break from it and relieve the stress. If the other fish is out of sight, it's out of mind for the fish chasing it. Especially with guppies and platies. They're not known as the brightest fish! They usually go well together and will likely settle, but if the tank is very open so the fish are always exposed (as often happens in new tanks, where plants haven't grown in yet), then they are always in view of the dominant one(s). Having things to break lines of sight helps.
In general, live plants will benefit the fish and tank in many ways, and don't have to be difficult or too expensive. floating plants are especially good since they're great at sucking up ammonia faster than the filter bacteria can process it, so they improve water quality. Crucially, they also add shade and shelter to the fish, so they feel safer and less stressed. In the wild, being exposed in open surface water means being more vulnerable to predators. They don't know there are no predators here! So some Amazon frogbit, savlinia, water lettuce - any floating plant
except duckweed will help all the fish feel less stressed, and help break lines of sight. Also easy to maintain - just chuck them in, and if they get too prolific and start to take over the entire surface, just remove as many as you want and bin them.
For lower down and the middle of the tank, fast growing and easy stem plants like limnophilia sessiflora, elodea, hornwort, guppy grass - any plant like that will grow rapidly and provide dense hiding spaces for the ones being chased, and for the fry you're going to have at any time! Those plants can also be planted or just chucked in and left to float -they don't care and will grow either way!
Good luck, and if you post a photo of your tank, we can make more specific recommendations.
