Cook bacon in large skillet on medium heat 10 minutes or until crisp, stirring occasionally. Remove bacon from skillet, reserving 2 tablespoons drippings in skillet. Drain bacon on paper towels. Meanwhile, add potatoes, mushrooms, peppers and onions to drippings; cook 10 minutes or until peppers and onions are crisp-tender, stirring occasionally.
Spread vegetable mixture onto bottom of 13x9-inch baking dish sprayed with cooking spray. Whisk eggs and sour cream until well blended; pour over vegetable mixture. Top with bacon and Velveeta.
Bake 40 minutes, or until center is set and casserole is heated through.