Hi Mike! The recipe likely calls for natural cocoa because it's calling for baking soda, so the natural cocoa is needed as an acid to neutralize the baking soda, a base. Baking soda is most likely called for rather than baking powder because of the acidity of sourdough. 

Cocoa is a super clumpy ingredient — it just wants to clump. So the lumps may have been cocoa that had absorbed moisture. Since you mentioned they were light in color, I'm leaning on flour or sourdough. Unless the egg whites scrambled, there's no reason for them to clump. 

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