According to The National Gardening Association, watermelons split while still on the vine due to inconsistent watering and hot weather. When melons become overheated or engorged with water, pressure builds inside that breaks the rind.
The National Gardening Association states that to prevent splitting in watermelons, gardeners should lay thick mulch around the plants to slow evaporation and keep the soil evenly moist. Mulch also helps keep soil temperatures steady. Straw mulch is best for melons, but even cardboard can be used in a pinch. For best results, gardeners should lay the watermelons on top of straw or cardboard when they begin to grow in order to prevent rotting.