Booting from external hard drive (Catalina to Mojave) issues when installing the old OS

I followed the steps here to create a bootable installation drive of Mojave https://support.apple.com/en-us/HT201372.

I am now able to get to the install Mojave screen where I am to wait for it to install. When it gets to about 1 minute to go, the laptop turns off, followed by the laptop turning on a black screen, the apple symbol comes up and just as it starts to load, the fan gets very load and the mac book shuts off completely. I then can only stop it from restarting and turning itself off endlessly by holding options and going back to the original hard drive. Very weird stuff - any ideas what's happening and how to fix it?

https://youtu.be/yol6Z8eIVaw