When the width of a river decreases, the speed of its current increases. The same phenomenon occurs with currents in your oven, except that, in this case, the flowing substance is air rather than water. If there is a narrow gap between a pan and an oven wall or another pan, the air currents that flow upward through it will move faster than those that flow upward through less restricted areas of your oven. And since a faster-moving hot air current cooks food faster than one that moves more slowly, uneven baking results.
** Asian Recipes