[{"_id":0,"categories":["a","b","c"]},{"_id":1,"categories":["d","e","f"]},{"_id":2,"categories":["g","h","i"]}]

db.collection.aggregate([{"$group":{"_id":null,"categories":{"$push":"$categories"}}},{"$project":{"_id":0}},{"$set":{"categories":{"$reduce":{"input":"$categories","initialValue":[],"in":{"$concatArrays":["$$value","$$this"]}}}}}])