In Quora there is an answer that states:
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"
Moreover it is forbidden, or at least advised against, in Islamic Law, and some scholars suggest that it is for health reasons.