Night time in the US (especially California) has always been a quieter time. I live in Europe and when I am regularly checking HN I can often tell when it's start of working time in the Bay based on the front page.

You can do a quick analysis but my observation is that stories on the front page during west coast night time are significantly more stale (submitted more hours ago) and/or with less points. Something getting near the top during that time with ~10 points is nothing unusual (especially if the upvotes happen this fast).

Edit: Also moderator intervention is a much more likely hypothesis than 'tricking the system' on average on HN, but this case is well within the norm so I wouldnt suspect that.