Good Question..
Scientifically, drinking while standing lets water hit the lower part of the esophagus strongly, & by time this leads to a dilation and relaxation of the sphincter that connects the esophagus and stomach, which finally leads to a medical condition called "GERD: GastroEsophageal Reflux Disease" in which the acids in the stomach pass backward through the "relaxed" sphincter and start "burning" the acid-intolerant esophagus. This leads to the sensation of heartburn that most of people complain of. And that's why drinking while sitting is much better for you.
ARTHRITIS
Studies show that drinking water whilst standing increases the risk of arthritis. Drinking water while standing disturbs the balance of other body fluids including the fluid in the joint spaces. It leads to the accumulation of excess joint fluid in the joint spaces. Clinically, this condition is characterized by joint pain, known as arthritis.
GIT DAMAGE
When we drink water while standing, it flows directly down the gut and hits the lower end of the stomach wall. This damages the stomach wall and whole GIT. If it is practiced for a long time, our GIT becomes elastic leading to the dysfunction of the digestive tract.
KIDNEY DAMAGE
Medical science says that intake of water while standing decreases its filtration from the kidneys. It may lead to accumulation of unwanted materials in the kidneys, bladder or blood which increases the risk of many diseases associated with kidney and bladder.