Well, first off, alexa.com is to be taken with about a pound of salt because of their way of sampling, even so, obviously something happened (maybe half the HN visitors decided to install the alexa toolbar).

Also, other such institutions can show wildly varying data, for instance have a look at 'compete' for ycombinator.com:

http://siteanalytics.compete.com/ycombinator.com/

It tells quite a different story.

The final arbiter here would be the log files of HN or some third party logging service (but HN doesn't use any of those afaik).

The traffic rank that alexa ascribes to news.YC is not in line with what I would expect at all (~5500), at that level HN should be seeing 150K uniques daily and I doubt that from the activity on the forum.