Arrived at the party late, so apologies if this has already been mentioned as a possibilty:

When the walker first collapses and the head hits the snow hard (perhaps an extra sound effect to reinforce the damage thats been done?) can you not instead show some energy 'sparks' around the entire head area (similar to force lightning) and this energy rapidly spreads to the main body which goes up with the aid of the snowspeeders firing upon it? This would then all make sense. Speeder pilot sees the possibility that the walker COULD blow from the energy discharges flashing around the head area and helps it on its way?

Nothing major would be needed to pull this off just some extra flashes around the head, possibly spreeding up the neck, and some extra sounds to help pull the illusion off? I'm sure Adywan could do this in his sleep ;)

- John