I feel like American Sniper was the biggest surprise. With almost all of the other films, you can make some sort of argument as to why they did what they did. Frozen had the Disney Animation and Disney Princess brand to it. Jumanji had an all-star cast. Deadpool was a superhero flick in the X-Men universe. Hunger Games was a popular book, and being someone who grew up during the peak of the book's popularity, I knew how popular the brand was towards teens. Even Blind Side had Sandra Bullock, and inspirational dramas like these have done well in the past.
With Sniper? It had little going for it, outside of Eastwood and Cooper. I remember looking at Box Office Mojo every day, and when I saw that weekend number, it made my jaw drop. It was an R-rated war drama about PTSD with decent but not phenomenal reviews, and here it was making superhero movie numbers, both in its opener and in its final total. The closest war movie as big as that film since its release is probably Dunkirk, and I don't think anything coming soon will come close to it.