Haha! Thank you! I actually just noticed that. I'm watching this page throughout the day: https://steemd.com/witnesses and this one as well: https://steemd.com/@lukestokes.mhth
It's kind of an addiction, actually. :)
Based on what I've seen is people powering down. When that hits, a big chunk of their vote weight decreases, which moves the rankings around a bit.