It may be that your eggs have been overcooked. When eggs are over-boiled, they will continue to get more watery the longer they sit in the hot water. Egg salad itself can get watery after it sits for a while, though. To counteract this, make sure the eggs aren't overcooked and chill them before chopping. You can also mix in a tablespoon of very fine bread crumbs to absorb extra moisture.