Having just watched Empire Strikes Back again with my sons, I noticed that Luke and his fellow speeder pilots are under heavy fire on every attack run.
The thing is: The AT-ATs appear to only have forward firing weapons with a small angle, and they are not fanning out to cover more angles. There are no anti-aircraft weapons shown anywhere in the scene. They are mostly untouched while circling the walkers or attacking from the side on one occasion.
This implies that after every run, the speeders regrouped directly in front of the walkers for the next suicidal run. Why did they do this and not attack repeatedly from the sides?
Note: I can imagine attacking from the rear was impossible due to the shield's proximity as well as useless due to armor, but the vulnerable neck is most exposed from the sides, and they were able to fly there in the movie.