What makes combustibility occur?

Answer 1

there's a couple ways, but mainly exposure to heat with isolated gasses. In simple terms: gas that's stuck gets hot. some things like lighters create a spark(a spark is created by friction, the heating up of things rubbing against eachother, like matches on strikers, usually using flint or some other highly flammable object) that comes in contact with flammable gasses and they catch fire. Other things may be heated to the point that they spontaniously combust.

Answer 2

Combustibility occurs when a substance combines with oxygen and releases energy in the form of heat and light. This chemical reaction, known as combustion, typically involves a fuel source and an oxidizing agent. When these two components are present in the right conditions (such as sufficient heat and oxygen), combustion can occur and result in a fire.