Egg salad might be high in protein, but between the egg yolks and the mayonnaise, it’s hardly a low-fat dish. In an attempt to reduce the amount of fat in the dish, some people will swap out regular mayonnaise for low-fat mayonnaise. However, according to home cooks on Chowhound, this could be the reason that your egg salad is turning out so runny. The Delicious Truth reports that the first ingredient in many low-fat, light, or fat-free mayo is water, as opposed to regular mayo, which lists oil as the first ingredient. So it makes sense that when you let it sit in the fridge, those lighter mayos break down faster than one with higher fat content.
An easy fix is to use the regular mayonnaise or, if you still want a lower-fat egg salad, some people recommend simply mixing in the low-fat mayo right before serving your dish.