This is becoming and increasing problem due to cheap cookware being sold over the internet. I have also seen this occur from metal which was previously recycled. If aluminum cookware is not coated or treated correctly, it will do this. Also it is recommended to NEVER use aluminum for high heat applications or acidic foods like tomatoes. Once the oxide layer is removed off the aluminum, all you're doing is poisoning your food.
Buy your cookware from a reputable dealer or use stainless steel, plastics, etc.