"However, at T+25s, a fuel leak of currently unknown origin caused a fire around the top of the main engine that cut into the first stage helium pneumatic system. On high resolution imagery, the fire is clearly visible within seconds after liftoff. Once the pneumatic pressure decayed below a critical value, the spring return safety function of the pre-valves forced them closed, shutting down the main engine at T+29s."
The turbo-pump also provides the high pressure kerosene for the hydraulic actuators, which then recycles into the low pressure inlet. This eliminates the need for a separate hydraulic power system and means that thrust vector control failure by running out of hydraulic fluid is not possible. A third use of the turbo-pump is to provide roll control by actuating the turbine exhaust nozzle.
Ouch! Being so clever with the hydraulic system may have hurt them on this- a lot more leak paths from the fuel system. If they could have survived long enough to get out of the atmosphere the fire would have gone out. B-nuts tend to loosen in a cryogenic environment. I'll bet they're reviwing their torque specs and their lock wiring.