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Say you're having a conversation with somebody. You keep explaining that you're making arrangements for tomorrow.
Then they make the comment "why you would do it for anybody else," but this is not based on any facts or any related circumstances.
So the comment is made as if this is something that happens all the time, when it's not, and there are no facts to back up the comment.
They have introduced a red herring into the conversation.
wikipedia Red herring:
A red herring is something that misleads or distracts from a relevant or important question
Cambridge has
Cambridge
Red herring:
a fact, idea, or subject that takes people's attention away from the central point being considered
If you like to use a Latinism, you may say that they have made a non sequitur
Merriam Webster
non sequitur:
a statement (such as a response) that does not follow logically from or is not clearly related to anything previously said
We were talking about the new restaurant when she threw in some non sequitur about her dog.
The person may also be facile (in its insulting sense rather than in the sense of doing things easily and with little effort)
Lexico
Facile:
Ignoring the true complexities of an issue; superficial
If the complexities are large, ignoring them is almost the same as ignoring the whole issue.
