What would cause horizontal ridges on fingernails?

Beau's lines: Beau's lines, which are grooves that run horizontally across the nail plate. They usually develop when nail plate growth, which begins in the nail matrix (located under the cuticle), is temporarily disrupted. This can occur with direct injury to the nail matrix; an inflammatory condition such as psoriasis; infection around the nail plate; repetitive picking at the nails or cuticles; or sickness.