Unfortunately, Queer has hit the nail here. Lots of Pashtuns and Punjabis take great pleasure in attacking Muhajirs.
Large areas of Karachi have been colonized by Afghans and Pashtuns, and are effective no go areas. Places like Sohrabgoth and Beneras are centres of all sorts of all sorts of criminal activity.
On Karachi, it is a Muhajir city, they built it, and make up the majority, but other communities are welcome, if they contribute. I generally find that Punjabis and Seraikis are good immigrants, they work hard, learn the local language and intergrate. Pashtuns, unfortunately form ethnic ghetoes, and refuse integrate. Hence the tension between the Bhumiputra (Muhajirs) and Pardesi Pashtuns.
Queer, in fairness you lot have the same problem in Mumbai, with the tensions between North Indians and Marathis.