Sometimes leafy vegetables such as kale or unevenly cut up foods such as onions can spark when cooking in your microwave oven.  It may be from dissolved nutrients such as iron, potassium and copper absorbed while the plant grew.  It can also be cause by uneven cooking when the small pieces burn and the large pieces are undercooked.  Below is more information on the science of sparking veggies and a guide on safely microwaving foods.

NPR: Why Vegetables Spark in the Microwave

USDA: Microwave Food Safety Pamphlet