It's possible the person who fired it thought the airliner was a US military craft. It's also possible it was intended to be fired at something else, but target

It's possible the person who fired it thought the airliner was a US military craft. It's also possible it was intended to be fired at something else, but targeted the airliner by mistake. Note that surface-to-air missiles are likely to target heat sources, and it seems like the engine of the airliner, a significant source of heat, may be where it was struck.