In Dug's Special Mission (the short following up), it has been revealed that the day Russell and Carl met Dug? It was his birthday, and he made a wish. What did he wished for? A new master. While it's a bit more Heartwarming in the events that followed that lead to meet the heroes, but considering how stable Charles Muntz's sanity is, it makes you wonder what the hell he did to Dug.
In relation to the above, it may possibly be due to how Muntz gets more dogs for his little canine army or how he gets rid of the people he kills under suspicion of stealing the bird from him. Mystery Meat dog chow anyone?
More than likely, considering while all the dogs pep up when he offhandedly says 'treat' during the dinner scene, he then gets them all to quiet down and back away fearfully when he says that 'having guests is such a delight.'
Er... Is nobody else concerned about how the Blue Umbrella blew onto the windscreen of a large truck, in the middle of a busy street?