In the context provided, the change that would likely cause crocodiles to lose competition for food is a drought. During a drought, water sources would diminish, leading to fewer opportunities for crocodiles to ambush antelope while they drink. This would reduce the crocodiles' access to their prey, while lions would still be able to hunt on land and may even have an easier time finding food as antelope may become more concentrated near the remaining water sources.
In contrast, wildfires, floods, and monsoons might disrupt the habitats but wouldn't directly reduce the crocodiles' access to food in the same way a drought would.