Google wrongly treats the topics tags as breadcrumbs, because Discourse marks them so

[neil (Neil Lalonde)]

@codinghorror

Except in the case when there are no categories, the “first” tag could be used. Which tag goes “first” is debatable though.. most popular tag by count of usage?

This is done. If a topic is uncategorized, then the most-used tag will be put in the title.