I am sure it is quite a complex decision. If I had to guess they have a bunch of document storage all over the place and they are unifying it.

As others have said, you can either validate the schema at WRITE time (RDBMS) or at READ time (NoSQL). Personally I hate the during READ time because then you have all sort of data integrity fun to work through... In either case, I am sure they have a ton of reference documents that they need to store, so easier to manage 1 Mongo than 64 different weird databases.