A mixed construction is a sentence with incompatible elements that begins with one type of structure and shifts to another type of structure. In these sentences, the speaker sets out to say one thing and abruptly switches to something else, resulting in confusion.
A sentence that is logical has a subject and a predicate. When a subject is introduced in a sentence, an expectation is set up about the grammatical direction the sentence is going in, and when that expectation is not met, the sentence does not sound right.