The rel="tag" is only included on question pages, not on pages listing questions. This includes your own site. The reason is the explanation from here:
By adding rel="tag" to a hyperlink, a page indicates that the destination of that hyperlink is an author-designated "tag" (or keyword/subject) for the current page.
In other words, the tag isn't accurate metadata for the questions listing page, but it is for the question page itself.