There are a few reasons why someone might gain weight while hiking. First, hiking can increase caloric intake because it requires more energy to move than walking or running. Second, hikers may eat more calories due to the increased physical activity and exposure to food and water sources. Third, hikers may consume more calories due to their increased appetite from being in an unfamiliar environment with new smells and sounds. Finally, people who are overweight or obese may gain weight while hiking if they do not adjust their diet and exercise habits accordingly.