North vs West Germanic: Why did they split?

How did the Northwest Germanic language split into North and West Germanic languages? How did these language families remain mutually intelligible? What were the differences between them? What foreign cultures influenced the language and culture of the Germanic folk? These are the sorts of answers I sought in this Jive Talk with the talented young philologist, Konrad Rosenberg. 
Germanic gems blog: https://germanicgems.substack.com/
Starting Heathenry: https://startingheathenry.thinkific.com

Thumbnail art by Linde Easterlingr