38 CFR §20.107

1. (a)General. A Member of the Board will disqualify himself or herself in a hearing or decision on an appeal if that appeal involves a determination in which he or she participated or had supervisory responsibility in the agency of original jurisdiction prior to his or her appointment as a Member of the Board, or where there are other circumstances which might give the impression of bias either for or against the appellant.
2. (b)Disqualification of Members by the Chairman. The Chairman of the Board, on his or her own motion, may disqualify a Member from acting in an appeal on the grounds set forth in paragraph (a) of this section and in those cases where a Member is unable or unwilling to act.