He has obsessive-compulsive disorder (OCD)
Everyone knows about Byakuya's OCD since he had showed signs that he had OCD many times in the past
Due to his OCD, Byakuya has frequent and uncued panic attacks.
It's also worth mentioning that whenever Byakuya has a panic attack, he ends up showing several signs that he's having one, allowing his friends to realize that he's having a panic attack rather quickly
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Byakuya's signs of a panic attack are:
Palpitations, trembling or shaking, shortness of breath, feelings of choking, chest pain or discomfort, nausea, hot flashes or chills, depersonalization (feeling detached from oneself), fears of dying
(Feel free to make a fanfic outta this if you want!)