Trigger warning for a lot of things: trauma, PTSD, murder, Nascour.
Have you noticed that Pregym has no trainer with fire or steel Pokémon with its student trainers (And eventually one psychic Pokémon in XD)?
Justy's students are very well aware that these types of Pokémon trigger something in the Preleader. They may not fully know the reason, but they know that those types make the Preleader uncomfortable. Anytime one is shown, Justy usually has to retreat to his office or to a private area to try and calm himself down from an upcoming panic attack.
Justy suffers from PTSD, ever since he was a child. When Cipher attacked his parents’ research center; a child Justy came face to face with Nascour and his Metagross, witnessing the Pokémon murdering his parents in front of him. Justy was still an inexperienced trainer and definitely stood no chance against Nascour. In fact, Justy became an easy target for Nascour to not only scare the child more, but to also physically beat up Justy in his frozen state by. The Preleader would have also gotten murdered if it wasn't for Ein coming with his Manectric to find the boy. Unfortunately for Ein this was his first encounter with a beta Shadow Pokémon, and was too beaten up by the Metagross. As a final attempt to leave neither witnesses nor traces behind, Nascour set the research center on fire, burning everybody and everything in it. Luckily for Justy, Ein and Manectric still had some strength left to drag Justy and his Sandshrew out of the burning building.
These memories haunt Justy to this day. Sometimes the boy will get nightmares on the event. Whenever the Preleader comes in contact with large open flames (i.e. a camp fire) he starts having a panic attack. Large psychic and steel Pokémon will also trigger his panic attacks. Only by hugging the stuffed rabbit doll Ein gave him can make Justy stable if his panic attacks are severe enough.