What causes VDW forces?
Electrons in charge clouds are always moving really quickly and at any moment they are more likely to be more to one side of an atom than the other. At this moment, the atom would have a temporary dipole
This dipole can cause another temporary dipole in the opposite direction on a neighbouring atom. The two dipoles are then attracted to each other.
The second dipole can cause another dipole in a third atom like a domino effect.
Bc electrons are constantly moving, the dipoles are being created and destroyed all the time. Even though the dipoles keep changing, the overall effect is for the atoms to be attracted to each another