Why do my arrows not fly straight?

So for a while now, I have had a number of on-&-off discussions with my fellow Korean Traditional Archery friends, regarding the topic of straight arrow flight. And the various reasons why the arrow might be flying crooked/sideways. Especially when shooting thumbring style archery, with warbow poundage bows. And while this video is a bit more geared towards Asiatic/Thumbdraw archery technique, specifically Korean Traditional Warbow technique. The basic concept still can apply to all forms of archery.

In my observations there are quite a number of valid potential reasons: