When I click a tag, my expectation as a sporadic user is to see a list of posts made using that tag. I'm willing to accept a popup that requires another click to get to posts with that tag, but when I instinctively click the first link my eye lands on (upper left corner) I will unfollow a tag I'm already following, instead of getting to a list of posts.
Eye tracking studies have shown that nearly all Web readers (except those whose first languages are read right-to-left) will look at the upper-left corner of a new page/tab/window first, and design in response to that user behavior should prioritize what the user likely wants to see by placing it in the upper left--for example, online news headlines are frontloaded with the most important 3-4 words starting the headline, so readers instantly know what the story they're looking at is about.
The most natural expectation when clicking, say, #askhubski is to see #askhubski posts. Therefore, either that click should go to the #askhubski tag's post list, or if it must generate a popup, the top left corner of the popup should have a link to #askhubski posts, not a link to follow/unfollow. It's unlikely that unfollow will be frequently used, so it doesn't need to be a prominent option for tags I'm already following.