Reactive hyperglycem. Its actually common for diabetics (type i particularly) to have increased glucose reading during or right after physcial activity. This phenomon is commonly known as reactive hyperglycemia and is due to the counter regulatory hormone system (cortisol and growth hormone) that get activated during stress or physical activity. You should watch for low bs however during the few hours after cessation.
Sugar from the liver. When you exercise it frequently lowers glucose, but many are surprised to see the glucose rise. This especially happens if the glucose is somewhat high when you start to exercise. Also the great american breakfast is frequently carbohydrate rich and glucose rises are a problem anyway.