Most chain reactions, such as the dominoes and forest fires, have to start with some potential energy. That means that energy has to be stored up some way. The dominoes are stood up, or the trees grow wood. It doesn't work unless this energy can build up.
Then some energy is required to start the chain reaction, we push over a domino or lightning strikes a tree. If it takes only a tiny amount of energy to start, it is unlikely that a chain reaction will happen, because the dominoes will keep falling over before enough of them are standing to make much of a chain. This is like what happens when people are bumping the table. In a forest, lots of little fires over time can prevent enough fuel building up for a big fire.
Once the first domino is pushed over, it releases more than enough energy to push over another, and that pushes the next, and so on. Once a tree starts burning, it can release more than enough heat and sparks to start its neighbors on fire. Something to experiment with is can you make one domino push over two more, and make those push two more each?
