The probability of a missile is still way overrated in this thread. Yes, it's a distinct possibility that the plane was shot down, but the likelyhood is far smaller than many here make it out to be.
In the history of aviation, only a small number of civil airliners
were actually downed by missiles. It's an extremely rare occurence. Technical incidents, on the other hand, have caused thousands, probably ten thousands of fatal crashes.
People put a lot of stock in the fact that the crash coincided with military action by Iran. That's certainly notable, and it raises the relative merit of the missile theory. At the same time, let's not forget that in many places in the world, both current and historical, military tension and civil aviation exist side by side. In many places of in the Middle East, for instance, there is a constant possibility of bombing / missiles, and yet passenger planes land and depart all the time without being accidentally shot down. During the Cold War, tensions were often extreme and air strikes or missile assaults were something many countries reckoned with or prepared for, and yet airliners were almost never shot down (but came crashing down for other reasons all the time).
I disagree with your argument.
1. This happened just hours after Iran had retaliated against the Soleimani assasination by launching missiles into Iraq. At this point in time, the Iranians were on high alert, waiting for the American response. That means highly nervous soldiers manning anti-aircraft missile systems.
2. The aircraft flew outside of it's scheduled time. As far as I know, no civilian aircraft were supposed to be airborne during this period of time. This lends creedence to the missile theory.
3. Nobody is suggesting Iran deliberately shot down a civilian airliner. It was almost certainly an accident.
4. There is damage to the aircraft that could come from a radar proximity fuse warhead. If it was the "shotgun" type that produces shrapnel, it would fit with the damage seen. However, if it was a "continious-rod warhead", it also fits with the damage seen. Iran operates both BUK-M1 and Tor-M1 SAM systems.
5. The various videos available show larger, burning chunks of the aircraft seperating in air while the aircraft is falling towards the ground. If it was a "continious-rod warhead", that would make sense - because it chops the plane in various pieces. But clearly, the other type of warhead could cause damage to an aircraft so it disintegrates as well. Like with MH17 which was shot down by a BUK-M1.
6. If Iran shot it down by accident, they would likely never admit to it. Just as the Russians and MH17.
7. The data available shows that whatever happened, happened very quickly. This wasn't some slow developing fire or anything like that. It was very fast. As far as we know, the crew did not put out any Mayday or Pan-Pan.
So that leaves three options:
1. Accidentally shot down by missile.
2. Bomb detonated onboard aircraft.
3. Catastrophic failure of engines to such a degree that it instantly ruptures fuel tanks and sets the whole plane on fire within a split second.
.. at a time when Iranian anti-aircraft crews were on high alert and likely very nervous for an incoming American airstrike. During a time no CIVILIAN airliners were supposed to be in the sky in this area. The SAM operators would only expect military aircraft at this time. (In theory).