There are two main reasons. First, cooking with too high heat, or removing the cover and breaking the moisture seal. To prevent this, cook with medium heat until the lid begins to flutter or a water seal forms, then reduce heat to low until food is done. To check for the correct temperature for the formation of a water seal, spin the lid on the pan. If it spins freely, you have achieved the correct temperature.
The second reason is using a saucepan that is too large for the quantity of food being prepared, and insufficient liquid from the food in a large pan can cause scorching. Always use the correct size pan for the amount of food. A pan at least 2/3 full or more is correct, and avoid cooking over a high heat.