Why does moving objects in some videos flicker or distort horizontally
This is called interlacing. It's a video artefact left over from the days when video signals for television (and domestic systems) sent every other horizontal line, then filled in the missing gaps, every half a frame of video that was sent.
You need to deinterlace the footage before you edit it.
If someone else has already uploaded the footage incorrectly to youtube, you may be unable to deinterlace their footage, depending on whether the scale and frame rate of the video has been altered when they edited it.