You could have a small scratch in the paint inside the microwave oven cavity. The paint is a protective covering to the metal which makes up the microwave and if the paint is chipped, even a tiny one, the microwaves react with the metal to some degree. If the scratch is large enough you will get sparks, but if it is small it may only blow fuses.
It could be any number of things from the internal wiring has melted onto something or a spillage has happened and got into some electrical component of the cooker without seeing the cooker its anyones guess. Suggest calling the Gas Safe appliance repair man in.
defective ignition module
