The system itself. It seems like the hydraulics failed because the computers on IFT-2 got destroyed in fires/engine failures

During ascent, the vehicle sustained fires from leaking propellant in the aft end of the Super Heavy booster, which eventually severed connection with the vehicle’s primary flight computer. This led to a loss of communications to the majority of booster engines and, ultimately, control of the vehicle.

https://www.spacex.com/updates/index.html