This looks like unusual behavior.

For example with a document like:

{ "_id" : ObjectId("4fd2736696a0c5567029c674"), "x" : 10 }

Issuing an update with $pullAll on a key that does not exist inside an embedded doc causes the field to be created:

db.test.update(

{x:10}

, {$pullAll : {"test.a" : ["foo", "bar"]}})

The (empty) embedded object "test" will now exist in the doc:
> db.test.find()
{ "_id" : ObjectId("4fd2736696a0c5567029c674"), "test" : { }, "x" : 10 }

I would have expected no change in the document's fields if the $pullAll references a key that does not exist.