see (c)
Rule 2.11: Disqualification of a Judge:
6) The judge:
(a) served as a lawyer in the matter in controversy, or was associated with a lawyer who participated substantially as a lawyer in the matter during such association;
(b) served in governmental employment, and in such capacity participated personally and substantially as a lawyer or public official concerning the proceeding, or has publicly expressed in such capacity an opinion concerning the merits of the particular matter in controversy;
(c) was a material witness concerning the matter; or
(d) previously presided as a judge over the matter in another court.
https://www.americanthinker.com/blog/2019/11/chief_justice_roberts_now_dragged_into_democrat_impeachment_process_.html