https://www.cnet.com/news/eat-your-burgers-upside-down-food-experts-say/

One of the main reasons for this flip-side way of eating a burger is that the crown, or top of the bun, is usually thicker than the bottom, or heel. 

The thicker, top part of the burger bun, food experts say, can better withstand all the meat and vegetable juices and condiments than the thinner bottom of the bun that gets soggier more quickly.