Intriguing.
I'd like to know more about how you derived the distance measurement.
I'd also like to hear "they all do that" from more than one source.
Given that this was in the cruise phase, probably on ap, it infers that the ap and/or yaw damper are over reacting, trying too hard to remedy a natural oscillation.
If the fuselage structure is compliant/flexible/damping enough not to be doing the same to the front I would certainly be inviting the very front row down to the back to experience it for themselves.