Crust cracking after scoring

I made a wheat bread using half all purpose flour and half whole wheat. This was a high hydration dough. Didn't knead too much. Did a series of S/F and then left it to rise for an hour. All turned out well till the baking stage. I slashed the dough. Baked in a cast iron covered with steam for 15 mins, then opened the lid and baked for 15 mins more. 

Light airy bread but the bread cracked in more places than where I slashed it. only on the top. No cracks along the sides. Would love to know why it happened. no flying crust.