Good point - the "overpressure" terminology suggests something else happening than a simple failure of a line or weld during normal chilldown.
The 2nd stage O2 tank is pressurized with helium during the chilldown of the engine lines. The helium tank pressure must necessarily be much higher than the nominal pressurized o2 tank pressure, so some regulation system must be used to step down the pressure. What if that system were to fail? Is the helium tank pressure enough to rupture the o2 tank if it were introduced unregulated?
Lets say it so: In the worst case, easily. Would a gas bottle in the tank burst due to manufacturing faults (which can always happen), you would also have shrapnel flying around in the tank, which can easily punch through the thin walls.
Also, you would have a very short time from first indication of a problem (overpressure event) to structural failure, about like observed. There are a few other branches in the failure tree thinkable, which would be very similar and which would only be a difference in the time between overpressure event and structural failure.
But simply a gas valve on a helium bottle stuck open would not be enough. Would multiple gas bottles stuck open, yes, that could overwhelm the vent system and pop the tank at its weakest location (tank dome welds usually)
But for any slow failure, remember that there had been almost no visible warning before massive amounts of LOX got released and got released at pretty constant rate for quite a time. So, the tank did not just explode completely, releasing all LOX in fractions of a second into the air stream and atomize it. There must have been a hole, much larger than the usual piping because of the much higher mass flow rates.
If the hole was on the forward tank dome, the inflow of LOX into the trunk of the Dragon could result in separation of either the Dragon capsule from the trunk or the whole spacecraft from the second stage.
Maybe the helium bottles also worked fine and simply opened automatically to keep the tank pressure constant whole the contents of the tank vented out through the hole. At one point, the hole will have grown larger and the tank pressure have dropped so far, that the tank itself will no longer resist aerodynamic forces and a rapid aerodynamic destruction sets in from above. Maybe something also triggered the FTS from inside. I am not sure there, but I would place my bets on the second explosion coming from the FTS.
But what is the root cause? A helium bottle? Maybe. But there could also be manufacturing faults, a falling IDA, etc. All could result in a "counter-intuitive overpressure event" as final anomaly in the telemetry.
I doubt that SpaceX will find an exact root cause there, even with 3000 values telemetered. But maybe they and NASA can limit the cause to 3-4 possible components of the stage.