probably being clumsy with the pump is causing it, but if you don't use a dustcap on the valve, it might be occurring at other times (cleaning, using a lock etc).
If you are doing it when using a Zefal HPX then you are probably in danger of tearing the valve out of the tube or other horrors. If your technique is that poor then should you attempt to use a pump with no check valve in the head (like an old Silca frame fit) you will be in danger of 'firing' the plunger out of the pump when you jam the valve open by twisting the head of the pump. It can go about a hundred yards or so. Not recommended.
FWIW the threaded parts in the valve stem locknut are usually made of brass; they sometimes break when you try and straighten them, so this is something to try at home, not by the side of the road.
cheers