Could have been bombed or shot down. Iran has their own domestic terrorist/freedom fighters (they oppress certain minorities, ie Kurds).

It was not the US if that's what you're thinking. We certainly wouldn't shoot down an airliner intentionally, and would have no reason to shoot down anything near Tehran. At this point I'd expect the kinetic drama to be confined to Arabian Gulf coastal regionals or the Iraq/Afghan border areas.

Or it might have just crashed, it happens.