I'm supposing that tags specified are reordered by occurrences (the most used fo first). The first tag goes directly in title of the question, not the others, even if they could be better related and descriptive. Why this choice?
They're sorted by popularity of each tag. The tag with the most previous questions becomes the main tag for the question and that's appended to the title.
I believe this is as much for search engine optimisation as anything else. It helps people on Google quickly see what the question they're going to click through to is about.
By extension, tags that already feature in the title text won't be prepended onto the title. The next tag available will be chosen instead.
None of the tags are included automatically in the question's title.
The title of a question is the text displayed immediately above question's body and voting arrows. It is also displayed in lists of questions and search results.
Often the most popular tag is added to the title of the question page, which appears on the browser window or tab for the question:
tag - question title - site name
The name of the site (Ask Ubuntu here) isn't part of the title of the question either.
Probably just one tag is included in the HTML title of the question page because:
- Including more than one tag would take up too much space and distract from the question's actual title.
- Incomplete information is better than no information.
The current behavior is inelegant, but I'm not sure it's worse than any other way of doing it. Maybe this could be a per-user setting, though (not so that you'd change it for your own questions, but so that you'd change how it's displayed to you for all questions).