What the wobble?

Ok, if there is anyone out there with some experience with turning bowls, can you explain this?

 Below the core and the mandrel you can see where I am chasing a bead down, and yet in just one part of the bowl the bead is sort of wavy, and only in that portion.  I was working hard to hold my tool steady, but maybe I was not working hard enough.  You can see it a little better below:

Was the mandrel loosening up and causing a weird wobble?