It's because even when it was being used to treat or prevent other infections, it also kills a lot of the campylobacter bacteria that are found in a chicken's gut and shed in their faeces - but not all. Some survive and this leads to strains of the campylobacter that are resistant to enrofloxacin, and related antibiotics like ciprofloxacin which is used in humans.
Campylobacter infections in humans are often caused by contaminated meat but can be picked up in other ways too, including through the faeces of a chicken kept as a pet. (Ever handle a chicken and then take out your phone to take a picture or something? You might've washed your hands after that but did you disinfect your phone too, or did you not clean it and then touch it again later on while eating a sandwich?) Exposure to tiny amounts like that might not be an issue for most people but could be really serious for someone with a compromised immune system. If the infection they pick up is resistant to antibiotics, it's not easy to treat.
Ciprofloxacin is also one of the few antibiotics used to treat other bacterial infections like anthrax. If antibiotic resistance genes were transferred from campylobacter to those other bacteria, that could be a huge problem.
Sir Jaffar is deeply implanted in my memory
