Easterhay said:
How come they were able to blow it up, though? "That armour's too strong for blasters."
They hit it in the neck. The neck is a weak spot as it's more like the corrugated passage between train cars than the train car itself, and therefore less heavily armoured; you'll notice that they also hit it in the head and it remains undamaged until they hit the neck - If I were to speculate, I would say that the lasers pierced through the neck and then through the deck and hit the motors for the front legs and that caused the explosion.
Targeting it while the AT-AT was moving would be more difficult, but once it was stationary... boom.
The AT-ATs are also weak from the underside.