Bingowings said:

That kind of doesn't make sense either.

How do they know that the neck is a weak point?

Until they shot at the things they have no idea that "that armour's too strong for blasters".

If they did know the neck was a weak point don't you think Luke would say, "Aim for the neck!"?

I'm sure the Empire wouldn't make it easy, but spinning a metal web, at speed around the legs of a mechanical dinosaur in motion belching laser bolts can't be easy either.

Almost every zombie film has someone telling the cast to aim for the head.

Every Doctor Who fan knows to aim for the eye stalk.

Every team of Ghostbusters has an "aim for the flat-top" guy.

Maybe the ground troops have been issued with a copy of Incredible Cross-Sections but the audience hasn't (the pilots certainly have not).

This is why I suggested cutting the explosion scene completely. It just doesn't add up. And all of the explanations take waay to much thought for such a quick scene. Having the walker fall over and then cutting to the rebel troops charging the battlefield is, in my opinion, the best option.