1. The main reason is that dogs are venting themselves, no companions, dogs will feel very boring and try to find a way to entertain themselves, barking at passers-by just can meet his needs.
2. Another reason is that during your absence, the dog takes the initiative to defend the family, and his bark is an alien intruder in budget
3. If the dog does not bark as he usually does, but he craned his neck to howl, the third reason is that he is gathering his companions.
Depending on the reason, you can take different measures to solve:
When you go out, find someone else to come home and take your dog out for a while and play with him for a while so that you can divide the whole day into segments and shorten the time the dog waits for his master to return home.
Give your dog some active toys to work with, either a cookie feeder or a pair of food-filled balls, but remember to deduct the amount of food from his daily ration.