The environmental change that would cause the crocodiles to lose the competition for food is drought.
In a drought, water levels would decrease, reducing the amount of habitat available for crocodiles and making it harder for them to ambush antelope. This would likely result in fewer successful hunts for crocodiles, giving lions an advantage in the competition for food. Other options like flood or monsoon could temporarily benefit crocodiles by increasing water levels, while wildfire typically affects land-dwelling animals like lions less directly.