When she and Reid unexpectedly came across serial killer Tobias Hankel in the episode "The Big Game," and split up to apprehend the suspect, JJ walked into a dark barn and was attacked by several attack dogs. Although JJ shot and killed the dogs, saving her own life, Reid was abducted by Hankel in the meantime, and would later be tortured and almost killed by Hankel before the BAU team was able to rescue him. After this traumatic experience, it appears that JJ developed a fear of dogs.