How is the display user for a question determined in the questions list page? It seems like whoever is the last participant in the question (asked, edited, posted answer), but then different views (different tag searches) display different user.
This is completely dependent on what tab at the top of the page you are viewing. Most views will only display when the question was asked. But, for example, if you go to the "active" tab, it will display when it was last modified, etc. I believe this is the only tab that actually does it, and it does that because they are being sorted by when they were last active, so that information is relevant to that list of questions.
Perhaps you got switched over to the active tab and didn't notice.
Edit: It also appears that all the lists in the Unanswered section of the site will display this way.