And some more...

The advantage of a blog, even that of a journalist, is that you do not have to follow all the conventions of the profession. I would not put down the following in print because it is based on unconfirmed, or rather unconfirmable (sic) sourcing.
This explains that the Congress decision to abruptly back off from the Indo-US nuclear deal and the confrontation with the Left was because it feared a coup. Had the Left declared that it no longer supported the coalition, it would have gone into a minority status. At this point, had the Prime Minister called for the dissolution of Parliament, his voice may not have held the necessary authority. (For the balance of forces and the arithmetic in parliament look here.)
Especially, if it was not unanimous within the Council of Ministers. The RJD (Lalu), the NCP (Sharad Pawar) and the DMK could have said they did not agree with the Congress. Neither they, nor the bulk of the Congress party, are hot on the nuclear deal, especially if it forces them to face elections right now. The RJD and DMK would have lost the bulk of the seats they currently hold and so could many Congress MPs who may have been denied tickets. Bird in hand....
At this stage had someone, say Mr. Sharad Pawar, said he would form a government, the fat would have been on fire. He would have been backed by the BJP and broken the Congress, his long-term ambition and created a right-wing third front with the help of the DMK with the RJD and the Samajwadis supporting from outside. (For the DMK's perspective, see this.)

While the Prime Minister and Sonia Gandhi were prepared to call the Left's bluff, they did not realise that their right flank was exposed. So, as soon as some inkling of this threat became apparent, they acted-- and in haste-- leaving the Left somewhat bewildered.
It is, of course, possible that if the Congress can keep the Left on board for a while and secure its right flank, they could execute their coup later this year, or in the middle of next year when a collapse of the government would lead to a general election, rather than a search for another government. What kind of a coalition dharma do you really expect in kaliyug ?