I'm not sure if we're able to change this functionality, but I don't think that retagging a question should make it move to the front of the site.
I believe the point of the tags are to help people find posts: 1. Through the search bar 2. Through search engines 3. Via their favorite tags preferences
and additionally to help people quickly understand a question's main topics without having to necessarily read the entire post.
With those use cases in mind, why should retagging a question make it move to the front of the site? It's not that any of the content of the question has changed.
The most immediately useful aspect of moving it to the front of the site is that someone with one of the newly added tags as their favorite will see it highlighted on the home page. Otherwise, having the question move to the front doesn't help when using the search bar, or searching through the search engines. In fact, it just hurts other newly created/updated questions by moving them farther down the on the front page.