Well now doesnt that suck. Still how is it that Samsung can't replicate the issue?
The issue could truly be limited to a specific subset of phones, and be something that is hard to replicate on demand.
Random hypothesis which fits the current crop of cases, but is not meant as an explanation - merely for illustration:
Let's say a battery manufacturer used an inferior or defective material as an insulator between individual cells in the battery - which can be done without even knowing it because their source may have sold them inferior product but claimed it was better.
Nothing that causes an instant failure - it looks and works great for a while - but something that degrades in a manner of weeks or months. Heat may accelerate this degradation and extreme heat may be somewhat associated with failures, but is not the root cause of it. Physical impact or vibration - even from tapping the screen - and even when the phone is completely off - may cause a degraded insulator to finally fall away and fail, but unless it's mostly degraded it's fine - so you can't associate it with that either. The root cause is something you cannot see degrading and letting two bits of metal touch, and there are a lot of things that will cause that final failure - but none of them are the actual root cause.
In any case, some (unknown) subset of batteries have this failure, which is as unpredictable as it is spectacular. And once a phone goes Thermite, it destroys all markings on the internal battery so you don't even have a way of knowing which lot of batteries it is.
Something like this would be very hard to pin down - especially if the batteries were used in a small percent of phones. You'd have to tear apart tens of thousands of phones, then tear apart the batteries in them, in order to determine the root cause. And then - if you finally did find a manufacturing run of batteries that did have this bad insulator in them - the information does you little good when you don't know which phones have the potentially-defective batteries (because you can't ask consumers to rip the back cover off and check a battery serial number - you glued the thing in).
The chances of finding the root cause of the failure are fairly low. The chances of the information being terribly useful are even lower. So the correct response is "recall them all - RIGHT NOW - and then when we have them all safely ensconced in a fireproof warehouse - THEN we can start tearing them apart and finding a root cause to avoid this from ever happening again." Because they are still flaming out, and Samsung has a huge liability issue at the moment.