Because vertigo.

From the opening titles to the bun in Madeleine's hair, swirly shapes are everywhere in the film. The camera swirls around Judy and Scottie as his mind goes spinning back to his memories of Madeleine. These swirls are like whirlpools. They signal danger. The policeman who dies trying to save Scottie falls to his death with his limbs splayed out in a spiral. The spiral staircase at the mission is a stairway to doom. The visual images all support the theme of the film—the dizzying distortion of reality when you're under the influence.

The influence of love, that is.