In each transaction, your application first checks that two or more doctors are currently on call; if yes, it assumes it’s safe for one doctor to go off call. Since the database is using snapshot isolation, both checks return 2, so both transactions proceed to the next stage. Alice updates her own record to take herself off call, and Bob updates his own record likewise. Both transactions commit, and now no doctor is on call. Your requirement of having at least one doctor on call has been violated. Characterizing write skew This anomaly is called write skew
