The "Event Reversal" that took place in this City did not proceed rapidly, minute by minute.
In real time, there was a noticeable gap in between.
After all, this effect came from Anton's "Traces of Time" magic, and one of the important conditions for its use was Anton's will, guiding the process from within.
(The principles behind this were explained in Chapter 788.)
So, from the moment Anton began surfing through the explosion's smoke and dust atop the back of Death Eater Mulciber's soul, until he had completely finished a cycle of the explosion, the entire City lay in a state of utter stillness.
The only living being left—well, half a living being—was Dumbledore's Wandering Spirit, kept alive by a Horcrux.
As for Old Voldy, he had been set to awaken at the Gaunt family home, guided by the Resurrection Ritual, and had been absent from this City for some time.
But clearly, Old Voldy had started making his move.
After all, he knew that if these people really found the box containing the Binding Magic, they would certainly target him alone. There was a good chance they'd send him, nuclear bomb and all, to the depths of some ocean and blow him to smithereens.
Although dying this time had always been part of his plan, dying in such a pathetic way would be utterly undignified.
As the saying went, even if you lost, you couldn't lose face. Apart from his inner circle—Barty Crouch Jr., Bella, Snape, Lucius, and the like—he sent all his other forces rushing toward New York. If they were going to play, they might as well go all out!
He even found a way to stop the Resurrection Ritual connected to the Gaunt family home.
And so, Dumbledore saw the scene he least wanted to witness.