There could be quite a few reasons, for example, you may have not drained the wet ingredients properly. If you add too many watery vegetables such as tomatoes, cucumbers, and onions, the salad might get runny. Try to balance the dish with a few watery and firm vegetables. Drain pasta thoroughly and let it sit in a strainer for a while. Take care of the consistency of the salad dressing and avoid making it too watery.