Well many experienced that Sindhi, Pakhtun and Muhajir rife was attributed to PPP, ANP and MQM's political conflicts and greed to control Karachi and the rest of Sindh.
MQM had the largest role to play. MQM's politics, headed by Altaf Hussain is based entirely on an ethnic agenda. You cant say the same for PPP (they get votes outside of Sindh, and have formed Govts in Punjab and NWFP), and ANP (which barely has a presence in the political scene in Sindh).