I looked at your video. What you are seeing is a very tiny sideways motion on a tiny amount of axial motion that has been magnified by a very large scale factor.
Your link had only the .wbpj file which is useless without the _files folder of the same name. Please create a Workbench Project Archive .wbpz file in the future to send models to other members. You can use the Attach button to put the file directly on your Post.
I assume you had one end fixed and the other end had a force applied in the X direction. Since you want straight line motion, you can put a Displacement support on the end with the force and set Y and Z to be zero while leaving X free.