Once word (instantly) gets around town that Lorelai has had a marriage proposal, all the townspeople are inordinately interested in seeing how Luke takes the news.
This is taken to exaggerated levels when a line of people, most of whom we have never seen before, form a line and begin following Lorelai to the diner. They then proceed to press their faces against the diner’s windows so they can watch Luke hear the news. They won’t be able to hear anything from the street, but apparently they don’t care.
For normal people, only friends and family (maybe) are interested in your wedding news; for Lorelai and Luke, they are the celebrities of Stars Hollow that even strangers find completely fascinating.
Rory tells Lorelai that “everyone knows” that Luke “has a thing for Lorelai”, which means that Rory knows too, and still discouraged Lorelai from seeking out a relationship with him.