I strongly suspect the range safety officer pulled the trigger. It did a bunch of unexpected tumbles, so I think if it were flight termination system, that would have popped it earlier. I don't think it came apart of its own account, the RUD was quite rapid.
The Flight Termination System is controlled by the Range Safety Officer. And yes, it was pretty clear from the way the two stages exploded that it was the RSO activating the FTS.
By "Flight Termination System" I meant some automatic process, if such a system exists. It might just be a big red button at this point (since the goal is data collection). or maybe it's a very generous flight envelope, but the tumble took it far enough off course that it tripped.
That would be an Autonomous Flight Termination System, which does exist (and is used on many newer rockets including Falcon 9) but I'd bet that a human pushed the button in this case.
