It can be a number of things, maybe one of these suggestions will shout out as to how to change the methods you are currently using to make potato salad.

1. Could be the variety of potato. Typically, an Idaho Russet Burbank has a high solids or starch content and is ideal for making a potato salad from russets. Look for the “Grown in Idaho” seal on the bag. The potato cells have a tendency to hold water or liquids better without bursting or breaking into mush.
2. My preferred method of cooking potatoes for a salad use, especially with russets, is to NOT boil or steam the potato whole. The outside gets overdone and will break down faster than the center of the potato which may actually be undercooked. Instead, peel, cut into even sized chunks 1 inch up to 1 ½ inches in diameter. These will cook, boil or steam faster. Test with a fork piercing the potato flesh so it gives without resistance.
3. Once the potatoes are done cooking, drain into a colander and then place the potatoes back in the pan on the stove for a couple of minutes to get rid of any excess moisture. Add seasonings while warm (but check again before serving as the potatoes will absorb some of the spices or salt and pepper flavors) and add again when cooled, to your taste preference. If mixing with mayo, wait till the potatoes have cooled or are chilled from placing in the refrigerator.
4. Just as with mashed potatoes, don’t overmix the ingredients or the potatoes will turn mushy or runny.