The rift has been there for centuries. Its not something new. The shia sunni wars and grudges go back to the early years of Islam.
In Pakistan the problem got exported from Iran after the Mullah revolution there in 1979
Pakistan has a long history of tit-for-tat attacks by militants from the majority Sunni and minority Shia communities and thousands of people have been killed, correspondents say. Source:
BBC NEWS | South Asia | Pakistan mosque blast ‘kills 20’