Why do Street dogs start barking after seeing Pet Dogs?

Someone asked if they jealous of these extra privileged Dogs and hence bark?
 

I think dogs protect their territories and bark at any new dog if it enters their area. So when the pet dogs come out and wander openly, these street dogs see it as a threatening act on their authority and hence bark and try to make them run away. Only my opinion; no primary evidence or research to back it up.
 

I think dogs don’t understand difference between pet dogs and street dogs. They would understand only “big dog/small dog/beautiful dog/fierce dog/hungry dog/strong dog/ etc. Street/pet classification is entirely political in nature which only humans are capable to do. 
 

Also, there is an interesting pattern when street dogs bark at and chase a running motorcycle! I read somewhere that motorbikes look like “giant dogs or some kind of freak animals” to the dogs and hence they try to put up a fight. So they see them as enemies in their territory. (this is also why they always “stop” chasing beyond a point – when their territory ends). Dogs don’t understand that bikes are not animals.
 

When a motor bike is moving, they see it as a kind of animal (because animals move). When motorbike is parked, it looks like furniture to them (as it is stationary). So they don’t bark at stationary bikes but only at moving bike. Ever heard the suggestion to stop and look stationary when attacked by a variety of animals and reptiles? It is also to confuse them that we are kind of furniture :) Or call it trees or wood. If they did not have this stationary /moving criteria they should bark at stationary bikes also which does not happen.