The people who created this used a "machine learning algorithm" to figure out who is most likely to die next. They go into a much deeper explanation about their initial data gathering from the books and how they use those factors to predict who's next on the website.
They've also got a ranking system to show you who you can expect to die, although these seem to be based on what the books have revealed so far so we may find out if these predictions come true when season six begins.
During a red carpet interview with The Upcoming, Maisie Williams got to talking about Game of Thrones and what's in store for Jon Snow (head to just after 3:00 in the video above to hear it for yourself):
Everyone was so sad about it, and I feel so mean because I speak to people and they're all like, 'Jon Snow, is he back?', and it's really heartbreaking because people are like, 'Yeah, but he's coming back though?' There's a great twist, but I can't say that he's going to be alive.
A statement she followed up with this face:
Well gee, thanks Maisie.
If you're a Thrones theory follower, you know there are quite a few possibilities being thrown around for what happens to Jon Snow after his apparent stabbing murder at the hands of the Night's Watch: that he'll be revived by Melisandre, that he'll warg into someone or something else, or that he'll become a white walker. Maisie's hint seems to imply the latter, which also fits awfully well with the series being called A Song of Ice and Fire. Daenerys has the fire pretty well covered, perhaps Jon can be the ice?