I am on disability as well for a MH diagnosis, under 65, and get asked this a lot. If telling them it is "due to disability" does not suffice and they try to probe further, I just tell them that, "I prefer not to (or do not) discuss it" and will "thank them for their concern though", even though I know they are being nosy, to soften it. This is something that is very personal and is on a need-to-know basis for me. I also do not discuss my other medical conditions either except as needed, so this is no exception. Find what works best for you with your comfort level in whom you disclose your information to as it is purely up to you, and truly isn't their business unless it is a part of your treatment plan, to help you with your care, or for your continuing disability reviews. Otherwise it is just curiosity and being nosy.