It makes no sense, but these silly chihuahuas eat the chicken food wherever they find it. Even raw rice! I'll let your imagination go to work as to what that does to a dog's digestive system. It's not pretty.
I think that it is just jealousy or that they are trying to show their superiority over the chickens. They don't have many opportunities because the chickens intimidate them.
Here is a video of the little monsters caught in the act: