If I am guessing correctly, a borrower has filed a complaint against an appraiser because the value was too low.
In order to validate the complaint, the borrower has sent in the OP's appraisal to the NCAB.
The borrower assumes that the low appraisal is the "wrong" or "bad" appraisal, because he just had it appraised and has since finished his basement, yet the value is lower.
The NCAB is not convinced the low appraisal is the "bad" appraisal and is taking a harder look at the original "good" appraisal.
The NCAB has hired more enforcement/investigative staff and will be taking a long and hard look at everything in your report and file, even the small stuff. For example, did you inspect the comparable sales? Are your cost figures supported?. Etc.
I would first look carefully over my entire file.
I would read my E&O policy. It may require that you immediately notify them of a formal complaint.
I would contact an attorney. North Carolina has an extremely qualified attorney that is also an appraiser, USPAP instructor, and former Director of the NCAB that specializes in defending complaints and suits against appraisers. His initial consultation will likely be at no cost. His representation will be expensive.
I would not take the chance at representing myself against a formal board complaint, even if I felt I had done nothing wrong. There is simply too much to lose: E&O rate hikes or loss of coverage, disclosure to current and potential clients of a disciplinary action, public record and Appraisal Bulletin posting of the case.
The best case scenario is that the case gets dismissed and a good attorney is probably the best bet to make that happen.