Since cold fronts are defined as a boundary of advancing cold air and cold air is more dense than warm air, the cold air creates sort of a boundary behind the front that looks kind of like a 1/4 of a circle. Because of its shape and density, the warm air is forced upward as the cold air advances creating many clouds. Typically these fronts are moving quickly so the uplifting can be fast, creating cumulonimbus clouds (usually T'storm/heavy precipitation events).