First of all, the clothes get burnt and then the smoke goes into the atmosphere and then it goes, to the global warning system which is also bad for humans ,animals and especially for the penguins and other cold environment animals.
It also affects the popular fashion brands. The customers would force the workers to make the clothes which look bad for them as then they're low paid workers (which is the fashion brands fault) make and the sell for more money! The clothes makers get paid a couple of pence per piece of clothing they make.
Its the fashion brands fault because they are the ones making the ideas of the clothes. It's also the customers fault(us)because we want cheap clothes shops like Primark.