They may not be cleaned out and have dirty, dusty bedding making them weeze, they probably are like humans, the other mans grass is always greener!!. I should try to stop them commin over by highering the fence with netting etc, best to keep them out anyway as they will eat the food you put out for yours!!! Some one else may have a greater idea of what they may have,
I am a real novice, just going by my instincts.