Most CMOS sensors do not photograph a frame instantaneously. They scan, reading pixel rows from top to bottom over a few milliseconds. If the subject or camera moves during that scan, the top of the frame records a slightly earlier moment than the bottom.
The artefacts are distinctive. Vertical objects lean when you pan quickly — the classic leaning-building effect. Propellers and rotors bend into implausible curves. Handheld footage develops a jelly-like wobble. And flash or strobe lighting can illuminate only part of a frame, producing a bright band.
Severity depends on readout speed, and faster sensors have narrowed the gap considerably. Practical mitigations are to pan more slowly, stabilise the camera, and avoid whip movements. The complete fix is a global shutter, which reads every pixel simultaneously at the cost of complexity and, historically, dynamic range.