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.
Yeah, most likely engine bay fire taking out systems one by one. Would be interesting to compare the telemetry cutoff with the video of explosion if possible. That could indicate if the fire even triggered an explosion, flight termination being activated or just reentry heating making the tanks explode.
Who knows where it started, but the fire was definitely in the payload bay in front of the header tanks if seen through the flap actuators during ascent, after speration at ~7:45 min