You had recently gotten into a fight with Kunikida. It was a dumb petty argument yet you took it too far. Kunikida remembers your exact words. They played in his mind over and over. "This is why you'll never be able to save everyone." It genuinely hurt him. He tried his best not to think of it but your words kept bugging him. This led him to spiral.

Your words eat away at him. He tries to drown out your words with work and work. He constantly works day and night, not eating, not sleeping, just working.