He had to at some point; and he's decided to do it now. Or else his boss has. For some time the government has been saying it would think about postponing again the 2p rise in fuel duty planned for first for April, then for October, and it's pretty obvious it had taken the decision already. The question was the tactical one of when to announce, or rather how long the government could get away with reassuring people with a mere expectation rather than an actual decision. Because now the decision's been taken, those who lead the fuel protests will want more concessions; there was an advantage in dragging out the postponement as long as possible so as to focus pressure on that one issue and delay having to consider yet more giveaways.
So why announce now? The timing is clearly linked to the Glasgow East by-election, where the SNP have been pressing the issue of high petrol and energy costs, whatever denials the government might attempt - making itself look silly in the process. There's also always a weather factor in these decisions, I think: what the government is trying to avoid is anything like the fuel protests of 2000, when lorry drivers (rebranded, for the purposes of this political debate, "road hauliers") got angry in the sun and threatened to bring the country to a stop. Street protests and riots often happen in the summer when the weather's hot (think Brixton and Toxteth) and so this time of year, especially if you think August might yet turn out to be quite warm, is a good time to get your concession in first.
It also looks as though Darling has concluded he needs to do yet more on this: the first BBC story I linked to earlier mentions that the government will bring forward further measures "in due course". If you've come to the conclusion that you need to go further than just the postponement, then you might as well trouser the political credit for that now and fall back on defending the slightly higher ground of what you're going to do next. A carefully planned rearguard action, this.
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