how does that prove that security concerns are due to sectarian differences? Let me tell you one thing being from Karachi, at the time of the blast in Ashura procession at Light House, Karachi, many Sunnis were also reported to be martyred.
The bomb blast in May 2005 at Sindh Madrsatul Islam happened , when Juma prayers were offered both by Shia and Sunnis simultaneously behind their respective Pesh Imams. So there is no active religious conflict among Karachiites. Politicians (majority so called specular) do play dead bodies politics day and night.