It's all about the ratio of ingredients between wet and dry. With more wet ingredients (eggs, butter, pumpkin puree), you need more leavening agents (baking powder and baking soda) to balance out the moisture in your loaf. You also want to get the breads out of the pan fairly quickly after taking it out of the oven. If the loaf stays in the pan, it risks getting gummy at the bottom. Let it cool in the pan for about 15 minutes on a wire rack, just until it cools enough to hold its shape. Then, turn it out of the pan and finish cooling the loaves upright on a wire rack.