Just found this on Drs Foster and Smith website:

Diet: Diet also plays a role in struvite formation. The urea that we mentioned above is formed when protein within the bladder is broken down by bacteria. The body's breakdown of large dietary proteins into smaller molecules also produces urea. Diets with excessively high levels of proteins simply provide the system with more urea to work with in the formation of ammonium and carbon dioxide. In truth, this may be the only factor in animals fed all-meat diets. Commercially prepared dog foods, even the highest protein varieties, would not be a factor in most animals.

Hmmmm...is California Natural that high in protein, or do I just have two dogs that are not most animals? Or maybe the food is not the reason.