I made a New York-style pizza using recipes from the Elements of Pizza book by Ken Forkish, and some of the cheese and pepperoni slid off in the oven while baking. This was my first time using these recipes and a baking steel.
I believe I followed the recipes pretty faithfully, measured everything by weight, etc. The steel preheated in my home oven at 500°F for about 45 minutes to an hour as the manufacturer recommended.
The book says to cook the pizza for nine to ten minutes. About five minutes in, I could hear something sizzling. When I peeked into the oven I could see the dough had puffed up, and some of the pepperoni and cheese appeared to have flowed over the edges in a couple places and was burning up on the steel.
My pizza went in the oven looking like this:
It came out looking like this:
Why did this happen and how can I avoid it?
Edit: added photos of second attempt with a smaller overflow.
Top edge where overflow occurred:
Close-up of top edge: