Makes sense. All the power plants need to be on the same page, frequency and phase wise. MASSIVE amounts of current uselessly flow from generator to generator otherwise, with enough current to fry the wires in very short order.
Systems are designed to shut down to prevent destruction of equipment. It takes time to re-synchronize and bring up the generators one by one, but nowhere near as much time as to replace all the windings one by one!
Power grids and equipment don’t take kindly to having parts of it out of phase. I knew a guy that paid cash for a new truck working an overhaul on a hydroelectric plant that got brought online severely out of phase (gross human error) resulting in an explosion of the armatures and field windings.
Pakistan might well be dark for awhile.