I and most believe that Nishter park blast was done by Khwarjee elements, who are killing sunnis (berelvis) all over Pakistan, and attacked many Milad-ul-Nabi gatherings (what was happening at Nishtar park too).
As for Sunni thareek, their political standing is irrelavant as far as Karachi is concerned. They are neither a threat to any Karachi political party then or now, that any political party would target their leaderships.