I hate to be the observant one in these things, but pay close attention.....
You never see the top of his shoe until the end of the trick.
If you look carefully, you see the loose lacings slowly go up the pantleg.
If I had to guess, the dude has another lace attached to a string or something going down his pant leg.
His shoe is already tied, and the loops are tucked under his pantleg as well.
As he starts to shake his foot and everyone is looking in that direction, he pulls the string, pulling the untied, seperate lace up his pant leg, while shaking the already tied loops out from under the cuff.
Watch it again...