Hi Chris! There are two common things that could cause this. One would be if the loaf is under-proofed and just has so much energy left in the yeast that when it hits the heat of the oven it essentially explodes, and it will find the weakest point of the dough and burst out. The other would be if it was just needed a way for steam to escape so it would benefit from being scored with a sharp knife, scissors, or a lame. 

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