I think I might have read somewhere that in extreme cases mania can produce word salad, but am not sure. It is more on the schizo spectrum than on the affective spectrum.

Why circumstantial speech, but not word salad, can happen in hypomania is easy to see - the hypomanic mind is quick to make associations, often perfectly valid, and those associations lead to digressions from, but then return to, the main topic of a conversation.