Occams razor for me - separatists with "found" Russian SAMs. They have already been shooting down transport planes. They see ML 777 and think it's Ukraine military transport and shoot it down. Oopsies!
Putin won't care, he won't reign those guys in. Just keep destabilizing Ukraine. The messier it gets, the more out of control, the easier to carve a chunk off and dominate the rest. And the longer it takes, the less the West will care.
Question is: what CAN the West do to show we care, and protect Poland and the Baltics (especially) from the same treatment? Will sanctions be enough? Until the Germans do something serious, Putin has a free hand and he knows it. And he is probably smart enough to not go too far to provoke them.
Too bad our leaders (in the US and the West in general) are politicians and not statesmen or strategists. Putin is playing chess and we are playing checkers. Worse, we have way more power (economic and military), yet Putin is running circles around us.