I've got to agree with ray on this one (welcome ray BTW).

As A kid I remembered it being odd that the Rebel troops broke the limited cover offered to them by the trenches to basically run up to the AT-AT just before speeders blew it up.

I imagine the intension was to have the troops throw grenades into exposed areas of the fallen walker (Luke later proved this was effective) but it was later found to be too difficult to blend the model and live action together realistically and wisely avoided a naff shot by having the speeders shoot some weak spot in the armour (I'm prepared to be corrected by you script experts out there).

It's clearly armour and not just force field blast repellent (that sounds like it should come in a spray can) they make a point of it in the dialogue.