As an Accusation
Rationalists are often accused of being rationalizers, and rational arguments are often accused of being mere rationalizations.
Any factual explanation necessarily involves a reasoned argument. If that reasoning is irrational in some way, then identifying the irrationality is a valid refutation of the argument. If no irrationality can be identified, then calling it a "rationalization" is not really justifiable.
Although rationalists (i.e. people who are trying to think and behave rationally) are quite capable of making rationalizations, the mere accusation of such is not a valid refutation and is arguably a form of rhetorical deception in that it distracts attention from the substance of the argument by interposing the emotionally-laden question of the speaker's credibility.
The only legitimate criticism of an argument claiming to be rational is to identify some part of it which is irrational or inaccurate.