Shift means a change.
It may mean a move, some movement that displaced a thing that normally was in one position, and placed it in another. But a train on a railway track in the middle of 6-hour journey is not shifting. It may shift tracks - move from one track to the other, but it's not shifting from 786th kilometer to 787th.
A shift in the movement means the movement changes. The tectonic plates moving 3cm a year for past millennium suddenly skipped 2 meters. That was a shift that caused the waves. The movement was building tension, storing energy, and once something snapped, the rapid change occurs - the movement caused the shift.