Police told Daily Times that the Taliban planted the explosives and then jumped over a wall of the shrine to escape.
While militant Islam is focused on imposing external compliance with Islamic Law, Sufism is focused on the "laws of the heart", an individual's internal state such that one does not defy God and actions are motivated by the of love of God, Islamic militant see Sufism as violating Islamic law when shrines of Sufi mystics become locations of prayer. Islamic militants see this as elevating humans to "consort" with God. In Islam, Allah has no consort. Traditionally this had been understood to dispute the claims of Christians that Jesus Christ is the Son of God. However, it is applied to any elevation of a human to supernatural status.
Robert Spencer at Jihad Watch calls attention to the US blog post out of North Carolina that celebrates the bombing as "great news", referring to those who visited the site as "shirk". This blogger includes this quotation on the post:
And of mankind are some who take (for worship) others besides Allah as rivals (to Allah). They love them as they love Allah. But those who believe, love Allah more (than anything else). If only, those who do wrong could see, when they will see the torment, that all power belongs to Allah and that Allah is Severe in punishment. (Al-Baqarah:165)
According to Robert:
Shirk = associating partners with Allah, i.e., worshiping others besides Allah.
It may also be that the Islamic militants objected to the women mixing with men at the shrine, as the issue of women was specifically mentioned in the letter.
Hat Tip: Women Against Shariah