It's likely because the potatoes absorbed too much water while cooking. This can easily happen with Russet or Idaho potatoes. Once the potatoes are cooked and drained, return them to the pot and let sit over low heat to evaporate as much water as possible. If your mashed potatoes are too runny, you can add cornstarch or potato starch (1 tbsp at a time) and continue cooking over medium heat warm over medium heat, stirring occasionally, until you reach the right consistency.