I've been developing stuff based on Bootstrap lately, and it amazes me how this article appears on the first page of many of my related searches - something like a third I would say. I don't know if this is due to the article's excellent content or due to some good referencing.
I've seen his blog before too. He has a lot of really good articles with clear, concise, and extremely useful writing. Most of his posts get lots of thoughtful comments, and presumably high powered links. I would assume this level of engagement+backlinking helps quite a bit with his search ranking.
I wrote the explanation above. Just good content on a topic that gets a ton of searches, but a lot of people don't write about it in clean, understandable ways. At the time, the Bootstrap doc didn't explain it in the same amount of detail. Now that google recognizes the content and the backlinks and social proof via HN, Twitter, etc, it's just on a snowball. It's at about 50k visitors a month, mostly from Stack Exchange and Google organic searches.
Once a blog gets a few top 3 ( and a few #1) hits in a google search, it just snowballs. I consistently get more and more visitors each day, more retweets, etc etc. Like after today, my average visitor count will jump up and then stabilize at a much higher average than yesterday, since once content like this that is instructional makes waves, people just link it and tweet it exponentially and it never dies off. Really crazy how useful content and targeted organic SEO can work.