Given that the engine telemetry shown on the broadcast showed the engines going out one by one over a period of some seconds, I could easily imagine some sort of catastrophic failure on a single engine that cascaded.
It could be many things, plumbing to the engines, tank leak, ect. You could see fire on the control flap actuators, so the ship interior was engulfed in fire at the same time the first engine was out.
Given the huge spread of the debris, it must have been a decent sized boom, no? I mean that's got to be 10's of miles wide in this video.
https://x.com/adavenport354/status/1880026262254809115
the flight termination system is sort of a shaped charge that's designed to rupture the oxidizer and fuel tanks. Even if only a few % fuel remains, it'll be a big boom.