Why did Sunak go for an early election?

The revelations in Graham Brady’s memoirs make the decision only more of a mystery

In an extract from his new autobiography, Lord Brady, the former 1922 Committee chairman, confirms that Rishi Sunak was not facing an imminent leadership challenge when he called a general election. 
He had also secured a dissolution from the King before consulting with the Cabinet. Leaving aside the constitutional propriety of such a move, it places further focus on the mystery of why Mr Sunak went to the polls so many months before he needed to.
A number of theories have been offered. Perhaps Mr Sunak thought he could catch Nigel Farage on the hop. If so, that backfired disastrously, and an election in the autumn would have seen Mr Farage in the US in any case. Perhaps he really did think there was no point in waiting because the voters would not engage with his arguments until forced to make a decision. But surely a rosier economic background would have made the stakes sharper still.
Or had Mr Sunak just given up? That would accord with his failure to consult his colleagues, many of whom had been intending to spend the summer campaigning in their constituencies. By the time the election was called, he was leading a fractious administration with no plan for how to address any of the nation’s challenges. His attempts to shift the polls had all failed. His signature ideas were uninspiring. 
There was little chance that a later election would have seen Mr Sunak storm to victory. But Labour won a large majority on a relatively low percentage of the vote. Tories will be tempted to wonder whether, had their leader chosen differently, their situation now might be rather better.