what can change the pressure in the bowls?

Answer 1

Foods that produces a large amount of extra gas, which may vary from person to person.

Answer 2

The question as written makes no sense, especially in chemistry. In biology, one could charitably assume you meant "bowels."

In any container, biological or not, pressure depends on the external environment and the contents and/or processes going on the inside.

Heat is probably the easiest and most common way to change pressure: add heat, the pressure increases, cool the environment and contents and pressure will decrease.

Changing the air pressure by elevating the container or putting it into a chamber that can evacuate the air will increase the pressure inside.

Increasing the air or solid content of the bowls (without removing anything) or the bowels will increase the pressure.
Applying physical pressure from the outside will increase the inside pressure.

Adding a gas absorbing material, like activated charcoal, will reduce the pressure caused by the gas.

Adding contents that interact and produce gas will increase the pressure.