Hello and welcome to BYC!
This could stem from simple pecking order issues and she is the lowest in the order. Make sure you have enough room in your coop and run...5 square feet per bird in the coop, 10 square feet per bird in the coop. Put out several food and water stations, even if you only have a few birds. Higher ranking birds can become aggressive with food and water, enough to starve them out. The higher ranking birds cant guard all feeders.
The flock can see things you cant. She could be sick and they are trying to run her off...flock preservation. Give her an overall exam looking for any clues that might indicate she is sick. Check for injuries, what color is her remaining comb, is she egg bound, what does her poop look like, has she been wormed recently, any respiratory ailments, etc... Treat any issues you find.
Lastly, you may just have an aggessive flock leader or others may be involved. Separation of one or more birds may be necessary, short or possibly permanently. Pinless peepers are an option for aggressive bird or birds.
