For many of Disney's recent live-action productions, it seems like post-production is at least a year-long process. I remember something very similar occurring with Maleficent. I read an article stating that filming had wrapped... but it was, like, 18 months before the release date! I'm sure it is a combination of some very tricky technical post-work with green screens, CGI, etc. I also think, as another poster mentioned, that it has to do with very deliberate release dates from Disney. They have had great success opening films within the spring break corridor (re: Cinderella), and want to replicate that release pattern. Other films opening in over a year (Pirates of the Caribbean, for example) have also wrapped filming.
It also isn't unique just to Disney. I swear Batman v. Superman finished filming, like, 2 years ago!