There can be a variety of reasons for a rabbit that is barbering. Hormones, sexual frustration, boredom, stress(eg. predators, stressful environment, environmental temperatures), nutrient deficiencies(not enough protein, not enough fiber, mineral deficiencies), skin irritation(mites, fleas, allergies, fungal, etc), pain from something internal - particularly if happening only in one spot(eg. arthritis, chest pain, etc), genetic, etc. If your two rabbits aren't kept together so it can't be the other rabbit doing the barbering or couldn't be doing it through adjoining pen bars; and it's your bun doing it to herself; it could be stress related, maybe due to the heat, or possibly still hormonal related, though this is usually more plucking and not barbering.
The one time I had barbering with a spayed rabbit, was due to a different batch of hay being a stalkier hay. My rabbit liked it so much that she ate more of the hay and not enough of her pellets. The hay was too low in protein, and she wasn't eating enough pellets to balance the low protein hay, so it created a nutrient deficiency in her diet. She was barbering due to not enough protein in her diet. Which was fixed by changing to a leafier hay(higher protein) and her eating pellets again.
So you'll need to examine every aspect of your rabbits life and environment, to see if there is a possible cause you can determine, or a possible trigger for the behavior. And if it starts creating a health concern, consult with your vet right away.
https://www.rabbitcaretips.com/rabbit-grooming-behavior/