For some reason it does not seem intuitive. Not sure if it is because when we are recording a person, they are normally standing up which is why we tend to record vertically, instead of horizontally. Regardless of the reason, you should ALWAYS record video when your device is horizontal.
There is a very easy trick to help you remember this. Think of a TV...a TV is more horizontal then vertical. Therefore, you device should ALWAYS be horizontal.
In fact, if you have ever used the app called Capture (made by YouTube) on your mobile device, you have noticed that the app will not allow you to capture any video unless you actually rotate the device so that it is horizontal. Below is a picture of what it looks like when the device is not horizontal.