Stupid Leap Seconds
MySQL didn't like leap seconds and that caused it to use up all the CPUs
MySQL didn't like it that we had a leap second a little while ago.
I remember hearing about a glitch in the linux kernel that caused things to bomb, but this is the first I'd heard about the same problem happening to mysql.
Solution found here: http://stackoverflow.com/a/11293475/423218
date -s "`date`"
Which I guess just forces the date to a point and gets it less freaked out. It's certainly made mysql a bit snappier, thankfully.