There is a lot of speculation around why the SAM site lit up on this airliner. Until the authorities have investigated or reported or until someone speaks with " anonymity because they are not authorized to speak officially" there will be nothing for us PPruners to examine, analyse or assess from which we can then meaningfully speculate. So it just becomes scenario creation. Here's one example scenario.
It might have been as simple as higher command raising the alert level to 'attack imminent' based on widely reported US military air activity directly associated with the ballistic missiles strike on their bases. EA6 Prowlers (see edit below) whose primary role is comms jamming were part of that I understand. On receiving this, maybe that was enough for the Iranian tactical command to announce 'possible enemy air or cruise missile attack imminent'. The SAM Site boys then go, "Ok, we're in business". These units then become fairly autonomous when in action since every time they light up their radar's to acquire a target they become a SEAD target themselves. The SAM site know that's what will kill them so if they knew they were giving away their position by lighting up an airliner and possibly risking almost certain elimination from the game, then I suspect they wouldn't have done so. Clearly, some or many of the elements of miscommunication, misidentification, miscalculation, fear, tension and C3 breakdown/misinterpretation were all at play. Comms jamming itself (which may have prevented the missile command centre receiving critical information concerning the Ukrainian flight), is an indicator of imminent enemy action and may have simply reinforced the 'under attack' scenario.
The inference is that civilian airliners simply shouldn't be anywhere near active military operations. The two cannot mix. No doubt some low level conscripts will pay a heavy price for the mistake (perhaps even execution in that regime), but the real culprits are the higher command and their failure to co-ordinate at the military to government interface. Such co-ordination would have simply banned civilian ops in Iranian air space thereby removing ambiguity from the missile battery's decision making. But who knows?
It is on their lives,that military in a 'hot' situation don't trust anything and as Lone Wolf has speculated they have very little time to react if they are to stay alive let alone defeat the enemy attack. It really doesn't matter how many civilian flights departed before the incident or what routes they took. Something in the threat level changed and this Ukrainian flight was in the wrong place at the wrong time.
As a matter of interest (and I am sure I am not the first to surmise this) any cruise missile taking an airways route might be well disguised where civilian operations are in play since their air speeds are similar (notwithstanding the slower climb out speeds of the incident aircraft). Conversely, it is very likely that every SAM site is well aware of that possibility and ready to neutralise any threat from that quarter.
Edit: Advised by Sailvi767 and Willowrun 6-3
"No EA-6 Prowler aircraft because their roles and missions have been handed off to the EA-18G Growler (a derivative of the F/A-18 Hornet)."
Last edited by Lord Farringdon; 12th Jan 2020 at 21:56.
Reason: Clarification re aircraft type.