The answer is that Jeremiah wrote the last six chapters of Zechariah and some post exilic forger tacked Jeremiah onto the end of Zechariah (which comes to a natural end at the conclusion of chapter eight) in an effort to turn the book of Zechariah in to a post exilic apocalyptic work. He succeeded and the Christadelphians fell for it. They think that Zechariah 12 - 14 refers to the return of Christ; which of course is nonsense.
The last six chapters of Zechariah are written in an earlier (pre-exilic) Hebrew style to the first eight chapters and they are full of imagery and themes used by Jeremiah in his book.
Matthew knew about the forgery and gave the correct name for the writer of Zechariah 11:13. "Then was fulfilled that which was spoken by Jeremy the prophet...."
That's what happens if you take a Christadelphian, shallow, unread, uneducated and naïve approach to reading and understanding the Bible. - You get it all wrong and wind up thinking that Christ is going to return.