Back in 2010, a clear message was sent from Pakistan to the Afghan Taliban in the aftermath of Mullah Ghani Baradar's arrest: ‘No deal without us.’ The pretext of this tough messaging was the secret negotiation process that was underway between the Afghan Taliban and the then-Afghan government being run by Hamid Karzai. It was bound to happen. Those who adhere to realism would endorse that states, whenever faced with a situation detrimental to their interests, react swiftly with little regard for legal or political repercussions.
Assessing the Outcome of Pakistan's Support for Afghan Taliban
An excellent read about Pakistan's Afghan Policy.