You make it sound like it was some dude in a foxhole snapping and firing his Stinger at a light in the sky.
No, it's a team of questionable training firing a 9М3(X) off what is probably an incomplete soviet missile system. Because they are rebels and their sh*t is probably busted up. It's likely they looted this weapon system from a military storage facility and shot the first thing they saw, trying to hit another Sukhoi. They could also have thought it was a transport plane, as they've shot down those in the region too.
They shot a cement truck at an airplane? I want to check out their launch system, because it's apparently badass. Yeah, that's what Google returned on '9M3(X)'. Anyway, Kiev source of questionable validity says the Ukraine military shot down the plane during a military exercise gone wrong.
Military expert says the operators might have put the system on automatic fire, and the airliner fit within the "valid target" parameters due to its size, so the thing just fired a missile at it all on its own like some Skynet ********* Or maybe the operator mistook the blip on the radar for a military cargo aircraft.
Either way, someone done ****** up.