There are many reasons why such as curves that are too tight, other AI drivers flipping track switches under long freight cars - this has been an issue since Trainz 1.0, engines with not enough pulling power breaking couplers, train too heavy, incorrectly set switches. Switches flipping back to their original position, on very long consists - this is a quirk but it does happen, and so many other issues. We would need to see videos and pictures to help troubleshoot something like this.
