 MISC: The Snow Speeder being able to destroy the AT after it is  tripped is often said to be an error seen as the blasters are  useless beforehand against the walkers. But from various sources  (including Andrew Darling in the Q&A section in an early edition of  the Official Star Wars Magazine) the explanation is that when the  walker is tripped the armour is damaged and unable to deflect the  power of the blasters. Also the pilots shoot at the much weaker neck  area where no protection is offered.