Why do certain noises cause pain in my ears?

At the grocery store today, someone behind me was pushing a very squeaky cart. It was awful; my ears really hurt and I had to cover them and run away. Subway trains sliding to a stop practically cause me to curl up on the ground in a shivering ball. But how can noise cause pain? I know that the vibrations in the air make the little hairs in the ears quiver, which transmits to the cochlea and thence to the brain, where it gets organized and presented as appropriately layered sound by the attention-deciding part. I know this is a hazy picture at best, but I think it’s more or less correct. What I don’t get is how nerve endings can be involved in all this – how do, and why would, the little cells fire off a pain-signal for things like chalk on a blackboard and the like?