wheel out of true (side to side).
Wheel has a lump in it (for example from a hard impact that blew out a tire and bent the rim).
wheel has a spot of grease on it
tire is not seated on bead, or has a bubble, or is somehow rubbing the brake pads
brake pads are not centered, aligned, or wheel is somehow losing contact with the pads for some other reason.
you should be able to find the cause visually. Spin the wheel, and gradually apply the brake.