Top heavy rocks like that would be in a state of unstable equilibrium - they might balance for a second or two but if you even breathed near them they'd topple.
There is a clue on his website. The temperature was minus 5 C. The rocks look wet. I suspect there is a thin layer of ice between the rocks which overcomes the tendency to topple as long as they are finely balanced and held there while the ice forms. He does say if it's windy, they come down.