It strictly seems like a bonus window with a 2 hour block chosen randomly. In my case for about 8 hours of farming, me and my friends been searching for these bonus blocks every two hours within the primal HM instances, which it self confirms that prior-activity doesn't affect your reward.
It's not only me and few other people who are converging into this "bonus window" theory, there are a lot of people contributing their information through reddit and BG forums. I'd like to think that simple answer is always the best answer.