Mohammed Yaseen, the chairman of the Pakistan Telecommunications Authority (PTA) stated that the ban was "done under the directions of the Ministry of Information Technology. It's because of blasphemous content."
"[The Ministry has] been discussing with [Twitter] for some time now, requesting them to remove some particular content," he said.
The content in question? Tweets discussing a contest in which caricatures of the Islamic prophet Muhammad were to be submitted. Unfortunately for Twitter, which refused to remove the "offending" tweets, representations of Muhammad are considered "un-Islamic" and blasphemous and are cause for censorship in the majority Muslim nation.
Twitter is a very popular service in Pakistan.