Not all x
It can be seen as a manifestation of guilt, or a misguided attempt at fixing the problem, in light of oppressive forces, e.g., white guilt. It also can be a reaction to being accused of intersectional oppression as a group or individual when one is already a member of an oppressed group, e.g., the reaction feminists have when transphobia within feminism is discussed in general, or specifically trans-exclusionary radical feminism. Furthermore, it can be seen as an example of the backfire effect.
Common examples include: "not all men" when discussing sexism; "not all white people" when discussing racism; "not all socialists" and "not all anarchists" when discussing misogyny and other oppressions in socialism and anarchism; and "not all feminists" when discussing racism and cissexism in feminism.