Debris-field dispersal has always fascinated me, in a morbid sort of way. I've always admired the professional debris-dispersal analysts. It seems at first glance like reading tea leaves. But these guys really do have insightful ways of plotting backwards from debris positions and conditions to initial-failure scenarios. You consider that heavy things like engine thrust chambers fall ballistically while drag-sensitive structures like nosecones are more susceptible to aerodynamic effects.
A second look at the skirt video convinces me that the engine we saw enveloped in flame was shedding hot metal. The "sparks" are a combination of hot metal being ejected and high-temperature impingement on nearby surfaces. (You see this in Apollo film of the Saturn V interstage jettison. As it ventures into the J-2 plumes, the impingement shows up as markedly incandescent.). But some points of light can't be explained by impingement. They are pieces of engine, piping, or structure being eroded by hot gas flow.
Since the kaboom happened so very shortly after engine relight, I'm guessing the sick engine failed. The problem with that is the plumbing. As others have pointed out, the Raptor operates at obscene chamber pressures. That has to be echoed in propellant-feed systems, which also have to maintain high pressure. If an engine separates entirely from the propellant feed system, you get a rapid loss of pressure in the propellant tanks. If the vehicle structure relies on pressure in the tanks, the vehicle is likely to rupture and release propellant. This is even more likely when the vehicle is under tremendous aerodynamic stress -- intentional or otherwise.
Another thing that can happen in high pressure engines is that once the feed pressure is gone, combustion fronts can race up the propellant lines and find their way to a more relatively enclosed component like a tank or an accumulator, resulting in a detonation instead of a mere deflagration. The smoking gun here is the methane header tank, which was found in two pieces quite a ways apart. This indicates that each half was on a different side of a high-energy event, like an explosion. So whatever the root cause was, I'm betting the kaboom was in the methane header tank.
