How did Grover almost get pulled into the pit of Tartarus?

Just a heads up, this answer has spoilers. To stay spoiler-free, skip the paragraph in the middle.
In Percy Jackson and The Olympians: The Lightning Thief, Percy is given a pair of magic winged shoes by Luke Castellan. The shoes were given to Luke by his father, the god Hermes.
As it turns out, long before he ever met Percy, Luke was convinced to join forces with Kronos, the lord of the Titans. Luke essentially became a pawn of Kronos. He gives Percy the winged shoes, knowing that they have been cursed to drag him down to Tartarus, the deepest and darkest part of the underworld where Kronos is imprisoned. This is all part of Kronos's plan to start a war among the gods; Percy is going on a mission to retrieve the Master Bolt, and if both Percy and the Master Bolt are dragged down to Tartarus, Kronos could use the weapon to break free.
However, Percy does not end up wearing the winged shoes. Grover does instead, because Percy is disliked by Zeus, and needs to stay out of his skies. In the Underworld the cursed shoes begin flying on their own, and almost drag Grover to Tartarus, but he is able to escape them.