i have had a silly thought of course the dwarves showing up was unexpected to bilbo. However it wasnt to gandalf. Bilbo gets upset when the dwarves eat him out of house and home. he says 'its as though they know my pantry as well as i do' i think they did and the feasting was designed so that when bilbo went on the adventure even though he left late he wouldnt come back to rotten food. I think gandalf told the dwarves what bilbo had (knowing hobbits better than anyone) so they would do just as they did and give one less reason for bilbo to object. notice how gandalf was waiting to get bilbo just before it would be too late. i think he planned that whole thing.