Too many variables in play to give an accurate "Root Cause".
It appears to be a harmonic, displacement of slack & inherent weaknesses.
Spindle nut preload could well be one place to look.
the natural tendency of the column to shake.
looseness in the any of the Gib adjustments.
Re Cutting of the chips can cause a harmonic skip pattern on surface, if any or more of the above is in play.
You can do some facsimile of my test cut, facing of all four sides, and increasing pressure to maximize faults and watch machine closely, even video all the week areas.
Note, that 2" of cut is quite a lot of pressure, and a good test in itself to find a weakness. Now, crank it up till you answer your own question !