It was a last ditch effort to create a media frenzy before the battle in Waziristan. The Taliban are on the back foot so they are going all out to kill as many and do as much damage as possible to be in the media spot light.
This is a last ditch effort by a group that is going to get WTFPWNED inshallah.
Lusi, they haven’t really “rejected” the proposal, they have voiced objections.
The reason why they attacked GHQ is that, army is fighting them in Swat and FATA. They have declared army as their enemy since a long time and this isnt the first attck on army, there have been dozens in the past few years.