So, one thing I noticed is that all the previous catches and controlled descents had a fuel rich engine shutdown with a fireball, rather than a bright green ox rich flame at shutdown.
I'd just like to point out to the many people pointing out the FTS activation that this is only the beginning of an answer unless you have an explanation for how the explosion drives an implosion.
The booster appeared to do an impersonation of the Titan submersible. Tank pressurization failed.
Not the first time we had a tank implode, in fact I believe the first high altitude starship flight failed when the fuel header tank imploded.
Edit: This very likely is a result of