Fractured clay has me Puzzled

If you look closely, this pot (fired on its side)
apparently has two vertical fractures in the clay.

Tall vase at the right has what appears
to be a similar fracture on the shoulder,
and the dish peaking out from under
the shelf on the ware bed has some cracks.

I'm about to open up the wood kiln completely and unload it, and I am excited and dreading it as well. In peaking inside, I've noticed several cracks and structural failures in some of the pieces. I'm wondering what I did wrong. Did I fire too quickly? Was my idea of firing up to 600 degrees one day and sealing it up and continuing the next morning from 470 degrees the reason? Did I begin reduction too early? Did I not seal up the kiln well at the end of the firing? Am I firing too high?

Anyone have any thoughts?