Telemetry continued through the event showing LOX overpressure spike on 2nd stage tank. Massive venting of lox captured on ground tracking video coming from just below the trunk adapter and second stage join point. Tracking video shows Dragon capsule on parallel track prior to final launcher disintegration.
Vehicle is traveling close to Mach 3 and well past the highest loading from punching a hole through the atmosphere. Was approaching the maximum G-loading as first stage runs empty close to 30 miles altitude.
Second stage engine was receiving cryogenic oxygen component-cooling per imminent staging procedure. Engine did “not” start causing loss of vehicle. A massive leak from the LOX tank of the second stage is evident.
Tank pressure is essential for structural strength of the vehicle body. Loss of pressure allows crumpling, like standing on an empty aluminum soda can. The capsule separated intact due to supporting structure failure. The atmospheric shock-wave loading, absent the capsule and fairing, collapsed and blasted apart the remaining vehicle structure.
I haven't seen a good description yet of the tank overpressure event. If it was gradual, then the stuck relief value idea seems to make sense to me. If it was very sudden (you mentioned a spike), then there may have been an internal combustion event as with the Apollo 13 command module's LOX tank. Or perhaps the tank was suddenly deformed or crushed from some external force such as a structural failure of the upper stage body.