It also didn't look like a breakup due to dynamic forces to me..., but can you give the source of that information? Thanks.
No confirmation yet, but the a number of factors suggest it. The press kit predicted a possible breakup either directly after separation or during reentry.
Following Crew Dragon’s separation, Falcon 9 is expected to aerodynamically break up offshore over the Atlantic Ocean. Expected breakup time will vary due toa number of factors, including winds and expected minor variations in vehicle attitudes and positions, but could occur shortly after separation or upon reentry from Earth’supper atmosphere. In either scenario, a dedicated team of SpaceX Falcon 9 recovery personnel will be staged and ready to begin recovering debris immediately after breakup.
The stage was flying stable after separation and showed no signs of tumbling or other instability. Even venting after engine shutdown appeared normal.
Also, destroying the stage controlled is a much better choice than letting it happen close to the capsule recovery site - but SpaceX has no control over the range safety devices anyway, the US Air Force has command there.
And finally the explosion cloud appeared too well mixed for a uncontrolled destruction. That is the weakest argument there, sure, but it is quite a gamble to get such a precise nice single explosion just by chance.
Its possible (in a conspiracy theory way) that SpaceX wanted to take some bit of gambling there to extract more engineering data for future landing technologies, while the Airforce was staunchly conservative and called for stage destruction as soon as it veered of the scheduled trajectory.
You'll get my excuse for the misinformation or the confirmation for the controlled destruction as soon as I have it.