Simple, because of HN's algorithm and population at this time of day. It probably got, due to pure random chance, a couple votes in quick succession, something which is rare at the start of the day, so it got promoted to the front page in case it's a smashing piece of news.

It'll decay over time, but sometimes this has the adverse effect of promoting not-as-interesting posts just because they had a chance of a spot on the front page, attracting more upvotes, while many very interesting articles never got enough time to be upvoted and are lost.

I quite like HN's algorithm, but this is one major flaw of upvote-based systems.