IME by far the commonest reason for this is the rust which forms between the outer surface of the wheel-hub and the inner surface of the brake disc which is mounted on the hub.
When the old disc is removed, which can be a struggle if its been there a while, it leaves behind an irregular surface, with pits and raised areas of rust. These must be wire-brushed away and any loose / flakey material removed. Next coat the outer surface of the hub and the inner surface of the new disc (after degreasing) with a thin film of copper grease and screw them together. (Note: this is usually a single set-screw that holds the disc in plalce; the real job of holding the brake-disc in place is done by the wheel nuts of the road-wheel, which is why they are so important.)
The hub / disc assembly must now be checked for true rotation (no movement in or out of the vertical plane) with an old-fashioned run-out gauge or equivalent modern laser device.
If these steps are not observed, you will experience the symptoms you describe. Some technicians will try to tell you that the cause is a warped disc (in years of driving trucks, vans, road, "performance" and competition cars I've never seen a warped disc) and use the run-out test to "prove" their case, but they will never show you the real causes - rust and laziness.