Reading some of these I think that many people have the same usage-pattern as I do, ie go to some aggregator site like HN and open a bunch of articles in one sweep and then read them.

So what happened was that the site opened in a tab, people didn't get to it immediately so when they got around to it the timer had fired and they were rickrolled.

It just navigated the page itself by assigning window.location.href, which isn't subject to the same restrictions as popups.

The page just refreshed. No new tabs.