The most prominent example of physics in this experiment is gravity. The gravity of the earth is working to pull the marshmallow towards its center, meaning you have to have an opposing force to keep it in place. For example, the spaghetti. Inertia is also working a bit here, because once the structure starts to tip it will continue in that direction because an object in motion wants to stay in motion. So, the inertia working with gravity pulled the marshmallow away from the center of gravity, and caused the marshmallow to fall if not held up.