The usual cause for this is inadequate preload on the swivel bearings. It may have been precipitated by the slight change in geometry when you replaced the springs on the front, and other factors can exacerbate the problem, such as steering problems, out of balance wheels, wide tyres, loose wheel bearings, loose u-bolts, loose or worn shackle pins or bushes. But the solution is almost always to correctly adjust the swivel preload. JD