This problem happens enough across the board with all recent STIs, especially on heavily ridden bikes, that there may be an element to it where the bends the cable is being subjected to is the real problem and there's some inevitability to it. But it's happening to you especially often. Here are some possible reasons:
- You shift a lot.
- The fancy nosed plastic Shimano ferrule isn't being used at the shifter.
- Friction elsewhere in the line, like from overly tight routing or any of the other causes for it that contemporary road bikes sometimes create, is causing you to need to be more forceful with the shifter, causing the first bit of cable to see higher loads on it.
- The cables have been getting installed in a manner that kinks them near the head. Then when combined with the tight bends that are inherent to this shifter, some strands are experiencing heightened fatigue and breaking prematurely.
Kinking the cable on this shifter during replacement is a fairly easy mistake to make, especially if you're not unwrapping the bars. The easy way to install the cable is to feed the whole thing through first until the end is resting in its seat, and then feed the whole thing back the other way, then into the tip of the ferrule. The trouble is if the ferrule is rigidly taped down and in place, it's easy to create a kink in the cable just ahead of it, especially with the nosed ferrules that one really should be using. The way to avoid this is feed the first part of the cable in and back the other way, but not pull it all the way through, and use a pick or small flathead screwdriver to guide it into the ferrule. Then gently push it through the housing bit by bit, and only seat the cable head in the shifter when it's all the way through and you can pull it into place without any of it getting bunched up and then kinking. Failure to do this is a way to cause the cable to break at the shifter.