Question

why do dogs bark on ragpickers?? Explain...

Answer 1

Dogs are highly dependent on their noses to assess the environment and do a lot of things that their forefathers the  wolves and wild dogs did.
Rag pickers smell different and they are taking with them something which smells familiar. 
Imagine a scene in forest in which a hyena comes in and grabs a dog kill and runs away.
Rag picker is the Hyena to the  dog pack. 
Hence the chasing away of the ragpicker.

Answer 2

Dogs are social animals. They understand the feeling of love and loyalty towards others.
But dogs are not as intelligent as humans to differentiate between useful things and garbage. Hence when ragpickers collect the waste, the dogs misunderstand them ad thieves and start barking to alert the owners.