The edge of the pizza is the first part to bake due to exposure to heat from three dimensions, top, bottom and the side where as the center of the pizza is only being baked from the bottom (not from the top due to the protective sauce, cheese and toppings). As a result of this the toppings tend to flow inward towards the center of the pizza. We used to instruct our students to build the pizza like a volcano, low in the center and higher out towards the edges (in reference to toppings). As the pizza bakes the toppings will flow towards the center. Since every dough is somewhat different in how it responds in the oven you may need to experiment a little to see what the ideal topping placement is for YOUR pizza. This issue is mostly associated with the thicker dimension thin crust type pizzas baked in a deck oven. Air impingement ovens do not show this characteristic due to the intense airflow to the top of the pizza holding everything in place BUT when a bubble forms in the dough the cheese and toppings WILL slide off of the bubble in any type of oven. In a pizzeria the common response is to pop the bubble and rearrange the toppings on the top of the pizza as it comes out of the oven.
Tom Lehmann/The Dough Doctor