Vertical video typically happens when recording with phones, which is becoming more and more common with the high quality of the lenses in smart phones.
In technical terms it is recording in an aspect ratio of 9×16 rather than 16×9, this makes playback on a 16×9 screen less than great. So when filming your video dabblings be sure to keep your camcorder properly positioned for the best possible footage.
This playful video discusses the problem, while at the same time making you smile. Enjoy.