In these post-Potter days (or pre, depending on your thoughts on The Cursed Child and Fantastic Beasts) fans have to find ways to keep the magical world alive. They can create fan fiction, work on their life-size replica of Hagrid's Hut, or delve deep into their rereads to come up with some amazing fan theories. Buzzfeed discovered a theory Tumblr user mynameisdavid23 came up with after rereading Harry Potter and The Prisoner of Azkaban.
He suggests that when Professor Trelawney, resident Divination buff, wouldn't sit down to Christmas dinner in the third book, thus making the table 13, and cursing the first one to die, she didn't know there might have already been 13 at the table. Ron's pet rat was actually a person, Peter Pettigrew, and he was often in his pocket. So when Dumbledore stood to offer her a seat, he condemned himself to death. And the "first of thirteen to get up" was a curse that followed (or, really, led) Sirius and Lupin to their graves.
If the theory is right, J.K. Rowling was giving us years to emotionally prepare ourselves for the demise of everyone's favourite magical principal. Not that we'd ever really be prepared for that.