Could be the bolts on the axle unthreaded loose, allowing the bearing cones to have too much play between the bearings. Or it could be it may have been tightened too much, or not enough grease. Hard to say without seeing it in person.
Try removing the wheel, repack the bearings with fresh grease and reassemble, but not too tightly or you might shorten the life of the bearings. Replace the tab for the coaster break or it WILL damage the axle. You can make one using a metal strap. I once made one from a strip I cut from an old license plate.