Why SWR depends on the impedance the feed line sees at the antenna, so anything that changes that impedance changes the reading. A loose or corroded connector, a broken braid strand, or an intermittent solder joint makes the connection resistance vary with vibration, wind, or heating, so the SWR meter jumps around unpredictably. Steady SWR that drifts slowly is normal (weather, ice, frequency change), but erratic, jumpy SWR almost always points to a bad mechanical connection.
Watch out Overmodulation is a transmitter audio problem that distorts your signal and splatters onto adjacent frequencies, but it does not change the impedance your feed line presents, so it does not make SWR jump.
Jumpy SWR = jiggly connection. Check the connectors first.