The chain on my 25 ton splitter snapped mid round and I still do not know why

I was splitting a pile of seasoned oak behind my shop off Highway 9 on Saturday when the chain on my splitter let go about 40 minutes in. It did not just stretch or slip, it broke clean and whipped back past my leg, and I stood there for a minute just breathing. I checked the tension before I started and it felt fine, maybe a hair loose if anything, and I had been running it for about 2 years without touching it. My best guess now is one of the links was worn thin and I never noticed because the outside looked fine. Anybody got a way to check for stretched links before they pop, or do you just swap the whole chain every season no matter what?