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As the bread cools it "deflates" slightly causing the crust to crack as it contracts a little. If you think what's happening when it's in the oven, the crust forms but the water in the bread is still heating and turning to steam which increases pressure on the crust, pushing it up and out. When it cools the steam condenses and the water is re-absorbed into the bread, reducing the pressure on the inside of the crust so it contracts slightly.
Three in a row now, but what's causing the crazy paving effect on the crust when it cooled?