What causes the crown of a bread loaf to crack as it bakes?
I would like to prevent it.
Disclaimer: I'm not too familiar with the english technical terms, but I'll try and explain what's happening without them.
Watch a shaped bread during the final rise:
If your goal is to get a non-cracked bread, you can put the loaf in the oven once it's fully proofed, just before over-proofed. This requires a lot of practise as you will basically have to monitor the bread towards the end of the final rising. You will also need to make sure that the loaf remains humid and pliable, you want as little of a dried skin as possible to allow for expansion without cracking. Poke the loaf all over with a chopstick or a paring knife to catch larger air bubbles that may cause the crown to burst locally and brush the bread with water before placing it in the oven.
Much easier is to bake at the second stage and plan for the expansion. There is a post here that discusses the influence of different cut patterns on the final shape and a bit more on the reasons for slashing here. Rule of thumb: the closer you are to "fully proofed" and the less oven spring you expect, the shallower your cuts should be. For a lot of oven spring, you need deeper cuts, usually made almost horizontally up to an inch in depth.
Should you be talking about a different kind of cracking, namely lots of small cracks that resemble the cracks on some glazed pottery and that appear during cooling: That is considered a sign of quality for some recipes.
The crown is the first thing that sets when you put the bread in the oven. Then, as the center of the loaf heats up, it rises and cracks the top, which has already set. I don't think you can prevent it completely, unless you change your recipe to a denser kind of bread that doesn't rise very much while baking. Otherwise, I find that you can minimise cracking by baking the bread at a slightly lower temperature, and placing a bowl of water in the over to add some moisture. Be warned though, both of these actions will change the final texture of the bread, making it denser.
What you are describing is the result of 'oven spring'. When a properly proofed piece of dough is placed in a hot oven it will begin to expand or spring. If the the oven is dry or the skin is not sliced it will set rather quickly, resulting in cracks.
There are two things that affect how quickly the skin sets:
Steam allows the crust to remain moist during the first part of baking so that it may stretch and expand.
Slicing the top of the loaf with a razor blade allows the loaf to expand further because it reduces how the crust constrains the loaf. A couple of diagonal slices is popular on long loafs. A cross is popular on round loafs.
If you increase the volume of a closed container which can't expand anymore, it will cause the container to break because there is no more space to expand unless the container gives way. This is what happens with bread. The top becomes fixed in shape as it looses moisture and looses its ability to expand. When the inside part expands, the lack of space will cause increased tension on the surface and at one point the surface will crack i.e. give way.
You can minimize random cracking by intentionally scoring the bread. This will help the gas which evolves to escape without causing expansion of the bread, thus preventing any cracks forming.
However you should be aware that this will decrease the volume of air inside bread thus the bread will be denser.