This article was very much about the data we've collected and our analysis of it, as opposed to our opinions as to why - had to keep it to a reasonable length!
So we kept that section short in the end. I do plan follow up posts that provide my theories as to why it's happening, and I think a best practices guide that discusses template-related XSS is a good idea. In the meantime, you can check out this related post: https://snyk.io/blog/type-manipulation/
What I learned only recently: With many modern javascript frameworks many of the assumptions you may have had about XSS in the past are obsolete. The strategies that worked in the past - proper escaping of untrusted input - don't necessarily work any more if you're using something like angularjs.