If it happens soon after you change tire and/or tube then I'd have to think you may be doing something wrong. Possibly pushing the bead of the tire over the bottom of the stem damaging the tube caught between.
If your valve stems don't come straight out of the rim and point toward the center of the hub, then there is something wrong that will cause the tube to wear quickly around the valve stem. If it's at an angle after riding your bike, then you likely don't have anywhere near enough air pressure in the tube and it's "walking" in the tire.
If you have a air chuck that locks onto the valve stem and when removed it pulls on the valve stem, well it's worn out and you need a new air chuck.
If you air up your tires with an air chuck you have to hold on the stem and are pushing the valve stem into the rim and bottoming out on the tire, then you are pinching and possibly damaging the tube between.
Regardless, if you figure it out let us know so we can add something else to check for.