The answer to 'what causes drip edge' is that drip edge is a metal flashing installed along the edges of a roof to help direct water away from the home's exterior walls and foundation. Drip edge helps prevent water from seeping under the roof's edge and causing damage to the roof, walls, and foundation.
Drip edge is typically made of galvanized steel, aluminum, or copper and is installed along the eaves and rakes (the sloped edges) of a roof. It extends slightly beyond the edge of the roof to create a small overhang, allowing water to drip off the edge rather than running down the side of the home.