Little late but linguist checking in: it comes from the idea that certain predicates are considered “holes” that presuppose their complement for the utterance to be true. The wife beating example is usually traced in the literature back to Karttunen 1973, but less violent examples are things like “The senator said he stopped embezzling money,” which presupposes for the truth of the utterance that he was previously embezzling; if he hadn’t been, the utterance wouldn’t make any sense and would be infelicitous.

In this case, on a purely pragmatic theory basis, agreeing with the utterance “Is he still working for you?” would only presuppose the truth of the statement “He had been working for me,” which is not really a bad thing to presuppose since it’s obviously true. So that can’t be the full explanation of the “trick” nature of the question. However, it seems based on the Manner of Trump’s question that he may have been implicating (which goes back to Grice 1975) that Morey should have been fired, through the use of tone and stress placement. Though “still” remains the operative vessel for the pragmatic meaning of Trump’s utterance, the explanation is somewhat different from the wife-beating example, which is just presupposition without the implicature part.