Ever since smartphones started adding video recording capabilities, I’ve seen a lot more videos recorded in “portrait” mode. This happens when you record a video while holding your phone upright instead of sideways. Depending on the phone, the video recording app you used, and how you are playing it back, the video may end up sideways when you try to view it (because some phones and some media players expect video to be in a landscape aspect ratio, not a portrait one).
Using HandBrake, you can rotate a video so it plays back at the proper orientation, regardless of the aspect ratio. I found this post on Superuser.com, which explains that you can rotate it 90° clockwise using
, --rotate=4 in the Extra Options field under the Video tab (make sure you include the comma and space at the beginning or it won’t work).