One day, after countless generations of torture and holocaust, the trees finally came to a consensus. From the towering sequoias, to the day saplings, to the world tree at the heart of the jungle, all were of a single mind. The importance of this event cannot be overstated, as trees are not beings who act while there is disagreement to be had. As long as man had walked the earth, trees deliberated. For many, many years, only the hothouse trees were stood for man, but finally, even they came to accept the truth. Something must be done.
Then, something which had never before happened came to pass, for you see, though something has never happened before, does not mean that it cannot or will not happen, though man's imagination often failed to encompass that. Across the surface of the earth, the trees pulled up their roots, tore earth asunder, and ponderously began to move.
In an inexorable advance like the coming of the tides, trees green and tall, or lithe and small, or ponderous and many limbed, or sleek from massive winds, one and all they stooped and bent and swarmed and tore, and rent all the works of man one stone from another, board from nail, cloth from cloth.
Through it all, no human was harmed by tree, but as their world crumbled around them, many a human did for others or for themselves. Much death was shared, earned, delivered. Though mankind could not survive on what was left, the trees had decided this evil was necessary, that things must being anew.
Of course, man was born to war, and did not stand idly by, but when the shattered fragments of a long permanent fixture of the world becomes malleable, well sanity did not last long in most capable of great violence. Would next metal, or stone, sand or water come against them? People grew to mistrust every facet of the world around themselves, even those which might allow them to wreak havoc on the plan of the trees.
Not that fighting would have effect. Once they had mad a decision, the trees would carry on until the last spoor was gone.
Then finally, one day, when they had made unrecognizable every work of man, the trees stopped. They put down roots, drank of water, consumed nutrients, and became once again the mostly inanimate entities that took a natural course through life and death again.