Could the issue be takeoff performance? Quetta would qualify as a hot AND high airfield
Yes, that could be the reason.
Last year, Hajj was performed from November 14 to 18 and PIA started Hajj flights operation on October 11. First pre-Hajj flight from Quetta was operated on October 12 with Boeing 777 aircraft, according to 2010 PIA Hajj flights schedule.
The year 2011 Hajj is expected to fall between November 4 and 9, according to info on the web, which means PIA will start its pre-Hajj flights operation around October 1, 2011 i.e. about ten days earlier than last year.
According to info on the web, Quetta Airport located at elevation of 5267 feet (Above Mean Sea Level - AMSL) has 12,000 feet long runway making it the civilian airport with longest runway in Pakistan.
Actually Quetta Airport has two parallel runways i.e. 12,000 feet long Runway 13/31 and 12,000 feet long Runway 13R/31L which is used for military aircraft only.
And according to information on Bing Weather
* Average temperature in month of September in Quetta is minimum 13° C and maximum 31° C.
* Average temperature in month of October in Quetta is minimum 7° C and maximum 25° C.
An airport located at an elevation of more than 5000 feet Above Mean Sea Level with temperature more than 26° C comes under Hot and High category, according to some sources.
Btw, if PIA decides to apply its logo on fuselage or tail markings on temporary basis for Hajj flights on leased TriStar or any of the other aircraft type listed in tender notice then it would make the leased aircraft in hybrid PIA livery a nice attraction for aviation spotters/photographers.