It's a quasi-random lottery. If the client-side code is similar to last year's, then the Javascript on the holding page polls the server at random intervals for an "available" ticket, which are released at random intervals and/or as orders fail to go through (e.g. payment times out, etc).

A result of PR people wanting to generate headlines such as the one in the OP. WWDC will sell out even faster, 'etc.