I assume the battery didn't…
I assume the battery didn't actually FAIL, but had lost enough capacity that the car could no longer do the job it was purchased to do. I'm sure Hyundai wanted it back to learn more about what happens to batteries when you work them that hard.
For the people that say, "but it normally costs $30,000 to replace a battery", how much does 400,000 miles worth of gas cost?