There will be many theories and would be glad to hear them all. My take is that there wasn't much (any?) strategic thinking (by him) in those days and he decided to give the Gilgit-Baltistan extremely mountainous areas to Pakistan on the basis that such areas would be difficult to live in, difficult to develop and also his native Hindu pundit community didn't care much for it, it wasn't their area of abode.
Your ideas and information please as to why Mr Nehru "gave" or decided to share the region according to that particular boundary as shown by the LoC.
If one observes the various actions of Nehru, there is no doubt that the conclusion reached will be that he had no strategic insight or vision.
Patel had, but Nehru's overreaching aura muffled the strategic insight of Patel.
However, to be sure, it was not Nehru who gave away Gilgit Baltistan to Pakistan.
Once Independence Act would be passed by the British Parliament, the sovereignty would pass onto the Princely states and the lease to the British Govt end.
The Maharaja of Kashmir sent Brig Ghansara Singh to go overland to Gilgit Baltistan as the Governor.
He was accosted and harassed by Maj Brown, the Comdt of the Gilgit Scout and Ghansara Singh was arrested. Maj Brown declared that Gilgit Baltistan has joined Pakistan.
The attack by the Pakistan Army organised and assisted with officering 'tribal' hordes along the Uri Baramulla axis, shifted the focus as the hordes had reached Badgam on the outskirts of Srinagar.
The rest is history that is known.
The Ceasefire sealed the fate of Gilgit Baltistan.