It is a special kind of ad hominem-- disqualify the speaker by eroding their earned trust.
And it is easy to do.
Trust is an important basis for communication, and it takes a long time to earn. There are strategies to build trust, and I teach these strategies to fifty audiences a year.
It starts with the Trust Equation. I didn't make this up, this is well known by psychologists and sociologists.
Trust is a perception an audience feels based on a speaker's competence, reliability, and intimacy-- divided by self-motivation or self-interest. Different aspects are targeted by those that want to harm a speaker's trust with an audience.
How do they do that? If you look at any online conversation where an expert is being criticized for explaining a subject of their expertise, you see opponents resort to attacking aspects of the Trust Equation.
1. Competence - It is very common for a scientist operating in public space to have their competence questioned. That is done in a variety of ways.
2. Reliability -
3. Intimacy --
4. Self-Motivation -- This is the big one. It is the basis of the notion that communicating science must be motivated by some other factor. Ulterior motives. There must be some reason that someone would want to communicate a scientific perspective beyond the reality that facts are our currency as scholars and we become teachers because we want to share them.
As professors our self-motivation is rather low. Our goal is a stronger society with an educated population, trained in critical thinking and situational analysis. We value independence and desire to not have others defining our research avenues. In return, we take jobs that require years of training for substantially lower salaries than we'd find in the private sector. Our participation in discussion of climate change, vaccination, genetic engineering and other topics is important because of the impartiality we bring, with minimal self interest.
In fact, self interest is generally so low to a learned scholar that they are willing to change their position when confronted with additional evidence. Pursuit of the Truth is the goal, whatever that takes.