If you aren't seeing any signs of drooling that could be causing irritation to the area(which wet or crusty fur around the mouth from drooling doesn't appear to be present in the photo), then there are a few possible skin conditions that can cause this. One would be fungal dermatitis and the other is rabbit syphilis. A bacterial skin infection may also be a possibility, but the first two are more likely to present with sores around the mouth like this.
Fungal dermatitis and rabbit syphilis can have a similar look at the start, so it can be hard to deterimine which it might be. If you happen to find bumps and lesions/crusting around the nose, eyes, or genitals as well, there's a good possibility it's rabbit syphilis. If it's just around the mouth at this stage, it could be either. For treatment, it's best to have your rabbit checked over by an experienced rabbit vet so the correct treatment and rabbit safe meds can be given.
Medirabbit: rabbit syphilis
Medirabbit: atypical rabbit syphilis
https://wabbitwiki.com/wiki/Rabbit_syphilis
Medirabbit: fungal dermatitis
https://rabbit.org/veterinarians/