There was a good piece on the Sky coverage explaining how it happened. From that, it can be seen why it was referred to as an 'unforced error', rather than a mistake.
In summary - the movement of his hand from clutch release to normal position means it passes over the button. In this motion, he inadvertently pressed the button.
Unforced error - a design feature that made this a possibility.
Mistake - a deliberate activation of the button by the driver.
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