Christians: Why do you believe after the rapture the world will be in turmoil?

Most Christians don't believe in the so-called "Rapture" (in its various conflicting forms). It is not genuine Christian teaching. The idea was dreamed up by a couple of itinerant Protestant preachers in the 19th Century. No Christian on earth ever heard of such a thing before then. the apostles never heard of it. Jesus Christ never heard of it.