Why White Liberals Get So Defensive
White liberals hold two major premises: the abstract proposition that racism is wrong, and the empirical observation that racism by whites against other races remains a serious problem in contemporary society. To apply these premises, white liberals tend to implicitly draw on a model of racism-as-character-flaw. That is, there are basically two kinds of people -- racists and non-racists. While not necessarily immutable, it requires something akin to a conversion experience to move from racist to non-racist. Racism is conceptualized as a feature of the person, not an effect of the person's behavior (including thought and speech behaviors) in the environment. In the racism-as-character-flaw conceptualization, moral importance is attached to "not being a racist," i.e. not being one of the bad people, rather than "not doing racist things."
Racism-as-character-flaw is a useful conceptualization for white liberals**. It allows us to define ourselves as part of the "good guys," the non-racists, while directing our energies to fight the bad racist other. Self-righteousness is more fun than self-criticism, after all.
This becomes a problem when a white liberal does something that helps to perpetuate racism, and gets called on it. The racism-as-character-flaw concept escalates an accusation of committing a racist act into an accusation of being a racist. After all, racist acts are committed by racists. The white liberal responds by trying to establish that they fall on the correct side of the racist-nonracist divide.
On the one hand, there's the external component to the accusation -- calling someone a racist, implying that racism is a fundamental part of their character, is a pretty severe charge. Once the criticism is misframed in this way, it's not surprising that the accused would become defensive, particularly if they have invested a lot in fighting those other racists. Thus a common response to being called out on one action is to list all the other non-racist or anti-racist actions one has done, seeking to show that the action in question can be dismissed as an outlier.
Perhaps more important, though, is the internal componenet. Seeing racism or nonracism as a part of one's fundamental identity leads to an ongoing sort of existential angst -- "might I really, deep down, be a racist after all?" So long as racism is seen as a character trait, this is a potent fear. A percieved accusation that one is a racist brings this fear to the surface. This is especially so since the racist act in question is inadvertant or thoughtless rather than a deliberate and conscious attempt to enforce white supremacy. We generally assume (with good reason) that unintentional acts are better guides to what someone is really like. To battle our own internal conscience, we direct defensiveness outward against the messenger.
*The analysis here I think applies to some extent to some other forms of oppression -- certainly sexism and somewhat to homophobia, but not really to class or fat-phobia, for example.
**It seems almost inevitable that whenever one group is the target of criticism (justified or unjustified), some members of the target group will respond "I agree that there are bad Xs and we need to crush them, but I'm one of the good Xs."