Loose connection in the antenna or feed line✓الإجابة الصحيحة
Over-modulation
Overload from a strong local station
الشرح
A loose or intermittent connection changes the impedance the line sees as it moves, vibrates or arcs, so the SWR reading jumps around unpredictably. Thunderstorms, overmodulation and strong local signals do not cause erratic SWR in this way.