Any animal with teeth can bite. That's the golden rule of working with animals. Juniper does bite occasionally. Rarely to the point where she breaks the skin like this, but it has/does happen.
Foxes are extremely moody animals, and anyone who has worked with a fox can tell you that they've gotten bitten like this at least once. They're very possessive of food and or toys and it takes a lot of training to get them to the point where this isn't a regular occurrence. Being able to read their body language and understanding their behavior is incredibly important.