Re: To quote Mr Manley...
More like, check your attachment points. Seems like one of the boosters might not have separated cleanly and slammed into the center core (as they always seem to do when I build a rocket in KSP). One of the core engine combustion chambers then seems to develop pogo-oscillations judging from the violent jerking seen in the video of the capsule interior (Phrasing!). Or one of the Vernier engine control systems was damaged and it was flapping about uncontrolled.
The pressurization tanks are also at the bottom of the core stage, so if that got damaged this could lead to pressurisation problems and irregular engine feed pressures resulting in aforementioned pogo oscillations.
(Interesting pic at wikipedia of the Soyuz construction: https://upload.wikimedia.org/wikipedia/commons/1/1d/Soyuz_rocket_and_spaceship_V1-1.svg)