This is one of those hyperspecific fallacies that we may not even need a distinct name for, because we can address all the issues that arise with it by means of other fallacies. You can think of Bulverism as an example of circular reasoning or you can think of it as an example of the ad hominem. What makes Bulverism unique is the special way it seems to combine these two more common fallacies into a single mistake.
The genetic fallacy is also in there somewhere.
Bulverism occurs when someone assumes that another person is wrong and sets about explaining why they came to be wrong. It’s easy to mistake this for a reasonable argument, because they are actually explaining the error, but what they are explaining about the error is the personal history of the individual they believe is making it. What they are NOT explaining, is what makes it an error in the first place. That is simply assumed.
The word condescension sometimes comes up in discussions of Bulverism.
For example; when I say that I think The Searchers is a great film, my girlfriend (Moni) sometimes says; “Oh, you just have a crush on John Wayne.” This is Bulverism. Moni’s response certainly communicates her dismissal of my opinion. She might even be thought to have answered the question of why my views on the movie are wrong, but only if we interpret the ‘why’ in this instance as a request for an explanation of my error rather than a request for a justification of the criticism. In effect, Moni is explaining why she thinks I came to be be so foolish as to think this a great film. She is not telling me on what grounds she believes me to be in error. Rather than providing grounds to believe the film is poorly done or refuting my claims for believing it is well done, she simply questions my motives. This is Bulverism.
…and she’s wrong about that movie!
Anyway, the term ‘Bulverism’ was coined by C.S. Lewis who playfully suggested he would one day write a fictional story about a man (Ezekiel Bulver) who discovered at a young age there was no reason to prove anyone wrong when you could simply assume they were wrong and explain their error. Lewis’ point was that people should only go on to field such speculation if and only if they had already shown those they criticize to be in error. Whether or not it is reasonable to comment on other people’s motives once you have established an argument to the effect that they are wrong is of course open to question, but the sort of argument Lewis labels here is certainly fallacious in its own right.