Changes in pH, nitrogen, herbivores, pathogens, shading, adding organic matter to the soil, etc. Sometimes species phase themselves out. Frequent in chaparral, grasslands, etc. Prairies in Illinois were stabilized by fire. In the last 500,000 years, coastal forests in California have undergone retrogressive succession.
Chance favors arrival of some propagules more than others at any given time.
Pioneers and many seral species and climax species all arrive, the last end up outcompeting the others in the long run. Elimination of early successional plots favors growth of older seres. BUT this idea is mostly based on a forest model. All of the pioneer species and many of the later seral species are present in the initial floristic composition. Some germinate and become established quickly, but others grow more slowly.