I have a set of Ultegra 6600 brifters (vintage 2008) and as part of my maintenance routine, I replace the wire every 9 months to a year. I'm guess it's a poor design, and the wire might be bent over a sharp point and as retilezs pointed out, the metal fatigues. I'm guessing if there is a smooth, radiused feature for the wire to bend over, it would eliminate the problem.

Best thing to do is replace the wire at some regular interval. And keep an extra wire or two in your tool box.