It is certainly not just normal wear. I think the normal life time of Dyform wire standing rigging should be >10 years. The fact that it breaks just above the swage for me seems to hint that it was either not made properly at the shop, or that the swage end has somehow been bent more than is advisable. The hard end of the swage will then act as a fulcrum, and give you a break right there.
When you are packing up the boat are you taping the stay down the mast or taking it off?
Are you rolling it?... maybe bending that end too much in the process?
In the Fireball I just recently replaced the shrouds after more than 10 years, since I also saw one of the strands of the dyform go... this was not at the swage through... but rather where the wire had an impact due to.. well.. an interesting capsize, involving the RC boat (grin).
All the best,