When you were plugged in last night and this afternoon, how were you plugged in? Official charger and cable? Plugged into a computer USB port? Something else?
If it was just plugged into a computer, that wou explain the drain. It's possible/likely that a computer USB port isn't powerful enough to charge the Maxx, especially if the display is lit.
As for the 8% anomaly, since there was a spontaneous reboot, I'd not be too concerned. It could be that whatever caused the phone to restart was preventing the phone from properly collecting battery statistics for some time so the calculations were off. I'd worry about it if you start seeing it again.
I haven't used Runkeeper on my Maxx yet. I tend to use the app only if I'm having problems with my gps watch, or if I forget to charge the watch, etc. I've generally found on both of my older phones that Runkeeper is less accurate than I want it to be. I generally ony need to measure how far I've run for routes I've never run before. But, that's me; I understand that others love the social and automatic data collection of Runkeeper. If you keep having problems with it I suppose it might be time to look at other solutions, like endomondo.
Or, perhaps the issue was slacker, and a different music service may be better.
