When two people have the same thought at the same time, as if it were telepathy. The reason for calling it a Mountain Dew moment is because the type of telepathy originated when two found girls both made the same stupid mistake when referring to caffeinated beverage. Instead of thinking about Coffee, Mountain Dew popped into their heads. Therefore creating the Mountain Dew moment.
Kathryn: I was looking at camp pictures a second ago.
Edith: WOAH GURL. SAME HERE.
Kathryn: MOUNTAIN DEW MOMENT.