It was probably some Russian hothead. A mistake. Still funny to watch the Russians twist themselves into pretzels over it.
It was either a mistake or a Chechnyan cat ran up on the BUK as a tech and messed with that laser targeting.
I go with mistake. Witness nobody upset about it, nobody up in arms, but Putin must pay nonetheless. You don’t make Airliners collateral damage and just get away with it with ‘oops’.