Ok...lets ask this question. Why would millitants attack GHQ when GHQ rejected america's proposal?
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.