On 2020 January 19 SpaceX launched the Crew Dragon capsule and tested an in-flight abort, where the capsule separated from the Falcon 9 shortly after launch.
Many headlines claim that the Falcon 9 was intentionally destroyed.
Isn't it more likely that the Falcon 9 was just left to its own fate after capsule separation, instead of intentionally triggering a self-destruct mechanism? After all, SpaceX was collecting telemetry from it until it exploded. SpaceX wouldn't have learned anything useful from destroying it before it fell into the ocean.
Secondarily, if it was left to its own fate, what caused it to explode suddenly? Will we have to wait for analysis of its telemetry?
A summary of recent comments and hints therefrom:
Quoting another answer, SpaceX expected
a Falcon 9 breakup... After thrust termination, the Falcon 9 trajectory would be uncontrolled and would be expected to start departing from the nominal trajectory. In this nominal scenario, the propellant is expected to be consumed in the deflagration or aerosolized. ... Off-nominal Scenario 2: Violation of autonomous flight termination criteria results in commanded destruct of Falcon 9."
Another answer, now deleted, surmises that
the Falcon 9's Range Safety System consists of explosive charges that "unzip" the fuel and oxygen tanks, allowing all the propellant to come together and combust rapidly in a huge fireball. The charges are detonated by command issued by... an autonomous onboard computer system.
Scott Manley's video analysis says:
"They didn't activate the flight termination system" (8:57 in his video). Had they done so, it would have "unzipped" and destroyed the second stage as well. But it was intact (SpaceX's video shows it from 3 to 10 seconds after the explosion; Scott at 9:18 shows a photo of it approaching the ocean), and it contained fuel and oxidizer that exploded upon ocean impact (10:22). At 9:48 Scott proposes that the booster split just below stage 2, tearing open stage 1's oxygen tank; a later rip in the fuel tank plus a hot spot would detonate stage 1.
So was the Falcon 9's destruction
automatically commanded by the AFTS automatic flight termination system, after thrust stopped and trajectory departed from nominal? (SpaceX's video shows that it didn't slow down much relative to its "contrail." Neither did its attitude change enough to call its flight sideways. But its ballistic trajectory must have dropped below what an engine-on trajectory would have been.)
uncommanded, and just a side effect of (for instance) asymmetrical aerodynamic loads letting fuel and oxidizer mix?