In those days the elders didn't back up their rules with science or logic. They used their stern demanding voices which were unarguably obeyed. But in today's digital age, kids are too smart to accept rules, and obey them , so they need to be logically explained as they grow up and start studying science as a subject in the school.
We should drink water from glass only and that too in sitting position. No straw is to be used or not even directly from bottle as in both these processes we tend to swallow air which also adds to constipation and flatulence. And in sitting position only to avoid the entry of water into the wind pipe or trachea. When we are drinking water in standing position we tend to be in a hurry. And drinking water in sleeping position is totally a no no as there is risk of aspiration. As we are conscious beings in our daily life we can cough and sneeze the aspired water out of the trachea but toddlers might not be able to do so completely. These reflexes are natural protective functions of our body.
Also, we should not talk while eating. We should chew the food with our mouth closed. these are also to prevent aerophagy.
Hiccups also occur when we eat or drink too fast and mix these acts with talking and laughing, as the swallowed air ( aerophagy) accumulates in the fundus of the stomach and irritates the diaphragm, thus stimulating the vagus nerve to start the hiccups. Through hiccups the gas is released bit by bit which becomes painful after 4 to 5 times. Usually we are advised or encouraged to drink water and initiate continuous peristaltic movements of the esophagus to force the trapped air to move down into the gut or else it gets released by belching.
So our grandparents were way more intelligent and caring than us, as they taught us the proper way to do the smallest and most natural activities of life in the correct way.