what caused the destruction in The Road?

The book doesn't tell you what the destructive force was on purpose, so there is no answer to this unless you like ask the author. I think the author intends you to imagine what it is for yourself. By not defining all the facts the story is made more relatable to a wider readership. For example, the book "2001 a Space Odyssey" is about a very specific story at a very specific time. Today, in 2010, that premise now seems a little more silly than it would if the book didn't specify a time. Not telling you how the world gets destroyed is a great touch to the story. After all, it is making you think about a little more.

But, in MY opinion I think it would have to be nuclear weapons. Nuclear detonations would have an incredibly bright flash and a very loud noise. Also, the book describes ash blotting out the sun and cold conditions, which are symptomatic of a nuclear winter.

I don't think it was supposed to be 2012 because the book was released in 2006 and all the hype about the predicted 2012 apocalypse became more well known only in the last few years.

So, my answer to your question is that the destruction is whatever you want it to be