So... I think there is a difference between asking what you do for work and why you are on disability. The work question is easier to answer and there have been many great suggestions. The "why" question is generally considered rude and intrusive. I am not, but if I were on disability I would answer the work question with something else and move the topic on. "Disability" isn't work anyway. How you deal with it might be. I think I would not disclose that I was on disability until I have gotten to know and trust them. Even then there is a risk they may not take the information the way you hope. In fact if you tell them you are on disability when they ask about work they may take it as an invitation to ask more about it.

Side note...Recently I have noticed a lot of people bringing up mental health conditions or treatments like dbt in social conversations. So it becomes pretty clear that I know more than the average person about mental health care. But nobody asks why I know so much and I don't tell them.