I think there could be many reasons; possibilities mentioned by various people include but are not limited to:
1. A genuine “Paknationalist” project that expects strategic victory in the region and feels the time is ripe to move to the next stage in pressure on the US.
2. The mercenary (or “pragmatic”) faction of the army has overplayed its anti-US propaganda and it is now out of control (in other words, they want to make a deal now but matters have slipped out of control)
3. Maybe the Americans are the problem. THEY and not Pakistan are raising the confrontation in order to break up Pakistan.
4. and of course, money.
This post is about point 4. I happened to talk to a friend who was a senior civil servant at one time. His take was this:
There is no strategic confrontation, no Islamist or Paknationalist project, no secret plan, nothing but money. The people in charge are people who know one thing and know it well. They can see a project a 1000 miles away and instantly gauge how much money can be made and how. They see Amrika leaving in 2014. They know getting out is harder than getting in and people are more willing to pay to get it over with (you can see this strategy in action at customs in India or Pakistan…outgoing travellers are routinely fleeced for trivial reasons because they expect them to pay instead of wasting more time in South Asia). They know what the bloody Uzbeks are charging. They think 5000 per container is a very fair price in the circumstances. And they think they hold the cards. IF they keep their nerve (and in these matters, they have nerves of steel) they will get 500o per container.
Most interesting, he was sure there are Americans (mid-level people used to milking third world projects) who will get a cut from the higher rates. And they are advising their Pakistani friends.
Longer term consequences of all this nasty bargaining? Strategic headaches? Islamist blowback? Who cares. A man of crisis makes his living from crises.
I think he may be overly cynical, but the explanation has the virtue of simplicity.