Some foods cause a quick raise in blood sugar levels and insulin response. The glycemic and insulin indices of the food you eat are scaled numbers that refer to how quickly a particular carbohydrate source enters the bloodstream as sugar and how much insulin is needed to rid that sugar from the bloodstream, respectively. Although insulin's primary function is to shuttle glucose (sugar) into skeletal muscle, it also carries many other nutrients to their respective storage sites -- this includes lipids (fat). Since carbohydrate ingestion, especially simple carbohydrates, stimulates a large insulin response and fat ingestion gives rise to blood lipid levels, when the two are consumed together, they promote the greatest fat storage.
Generally speaking, the quicker sugar enters the bloodstream, the more insulin is needed to rid that sugar from the bloodstream. When high levels of insulin are present within the blood, fat burning is slowed or even brought to a halt.