The phrase "stochastic events" refers to unpredictable or random occurrences. These events cannot be precisely predicted as they involve a level of uncertainty. Full definition
Environmentally stochastic events are random events, such as severe weather, floods, fires, and other ecological disturbances. (britannica.com)
Demographically stochastic events are random fluctuations in population variables, such as sex ratios and number of births or deaths. (britannica.com)
Remaining populations are mostly small and fragmented, thus vulnerable to stochastic events that reduce survival of breeding adults [2]. (journals.plos.org)