I don't think anything happened, except perhaps life. :-)
This site is a thriving place, but it's not anyone's entire life. Stuff happens. Priorities change. People need to take breaks.
I (not that I'm a 'power user') was all but absent for a few months, and now I am back. That may happen again, and it could have happened for some of the 'power users'.
The bottom line is that this community is comprised of volunteers. I like spending time here (and I'm sure others feel the same) because it's fun to solve problems and learn lots of new things. But sometimes things come up and "something's gotta give". Certain people not spending as much time here doesn't necessarily mean there's been a falling-out. (And nothing that I've seen indicates that that's the case.)
To address the other items in the question (some of this has already been mentioned in the comments):
Several users are known for being prolific Investors/Promoters, transferring significant portions of their Reputation into bounties to encourage high-quality content on the site.
One user (that I know of, maybe more) has a habit of changing his/her display name roughly as often as the new moon occurs.