It seems US intelligence was the driving force to roll up the UK bomb plotters this summer and threatened to take actions themselves if the UK and Pakistan did not move to stop the terrorist cells’ efforts:

The US warned Britain that it was prepared to seize the key suspect in the UK’s biggest ever anti-terrorism operation and fly him to a secret detention centre for interrogation by American agents, even if this meant riding roughshod over its closest ally, The Observer can reveal.

American intelligence agents told their British counterparts they were ready to ‘render’ Rashid Rauf, a British citizen allegedly linked to al-Qaeda in Afghanistan and who was under surveillance in Pakistan, unless he was picked up immediately. Rauf is the key suspect in the alleged plot to detonate explosives on up to 10 transatlantic planes that was exposed in August and, according to the police, would have brought ‘mass murder on an unimaginable scale’.

…

mmediately following the US’s veiled ultimatum that MI6 should ‘lift’ Rauf, which was communicated to ISI, he was arrested by Pakistani intelligence officials, a move that forced the British police to carry out a series of arrests as they looked to pick up those allegedly linked to him. Rauf’s father, Abdel, was arrested in Pakistan. Rauf’s brother, Tayib, from Birmingham, was arrested and later released without charge.

What this tells me is the US had some intel that was worrisome and they did not want to expose what they knew to the UK or Pakistan. The communication surveillance hints at involvement by our crack NSA. So what concerned the US so much that they pushed these other two countries into action? Who knows right now. But I would guess the intelligence community was not liking what they were seeing to pull the trigger so hard and quick. I gather the idea they would be US planes falling from US skys over US population centers could have had something to do with the feeling of emergency.