Natural disturbances, like forest fires or epidemics, can completely decimate a flourishing ecosystem. This then starts the process of Secondary Succession, as long as the natural disturbance left the soil. The forest then regrows, and this predisposes it again to natural disturbances. During the beginning of Secondary Succession, there wouldn't be many wildfires, because there wouldn't be enough fuel to spread it, so later on when the forest forms is when there would be wildfires, and so forests cycle through this many times.