WordPress has an extremely annoying issue with reading EXIF data when posting photos and videos. There is a plugin to fix rotated pictures, but I couldn’t find one for rotated videos. If you happen to upload a video from your phone that was shot in portait orientation, it will likely upload to wordpress in landscape orientation (sideways.) Very annoying.
My fix to this is to re-encode the video so it is at the proper orientation before uploading to wordpress. Thanks to this site for the information – ffmpeg does this automatically for you. So the syntax is simple:
ffmpeg -i SOURCE_VIDEO_FILENAME -c:a copy FIXED_VIDEO_FILENAME.mp4