Diction can refer to two aspects of a person's speech. It can either refer to the choice of words, or the way a person speaks, such as accent, intonation, or inflection. In light of these two aspects, unusual diction refers to when a person breaks the normal patterns of speaking. Here a a few examples.
First, if a person grew up in the street of New York and he stated that something was lower than a grasshopper's belly, it would seem odd. The imagery is not urban and most people in New York do not speak in that fashion. This would be an example of unusual diction in terms of word choice.
Second, if a person always spoke with a rising intonation, even in non-interrogative sentences, this would be another example of unusual diction. This pattern of speech would be particular to that person only, because a rising intonation should only occur in questions.
Many more examples could be multiplied. The point is that any break with the typical way people speak in a given culture can be classified as unusual diction.