IMHO, when a cham drinks too much too fast, they can overfill their stomach. They don't have a sphincter muscle at the base of the esophagus to hold back stomach contents if it gets too full and the cham moves around, they might spit some water back up again. To me, if your cham is so thirsty that it drinks this desperately, you need to check your cage humidity level and how often you are offering water.