Presuming that this was either a US carrier or a departure from the US, the carrier may not ask about the nature of the disability.
However, as you related the incident, it appears that you simply asked to preboard. Therefore "why" is a valid question.
The better approach is to simply advise that you have a disability which requires preboarding and leave it at that. If asked anything further, simply say that you don't care to discuss it, although in most cases that won't happen if you make it clear in your statement why you need the preboard.