The environmental change that would most likely cause the crocodiles to lose the competition for food is a drought.
During a drought, water levels in rivers and ponds decrease, making it more difficult for crocodiles to hide and ambush antelope when they come to drink. Additionally, as the available water sources dwindle, antelope may have fewer opportunities to drink, which would further reduce the crocodiles' chances of successfully hunting them. In contrast, monsoons or floods would increase water availability, potentially benefiting crocodiles by providing more opportunities to ambush prey. Wildfires could also reduce the number of antelope in the long term but would not directly impair the crocodiles' ambush strategy in water.