The machine failed once with grit in the bearing before even leaving Hitachi. That it failed again for the same reason should not be surprising. A little steel well casing might break teeth, but it isn't going to fill the bearing with sand.
"Preedy said that when Bertha was being built in Japan, one of the tests performed was to turn on the motors and rotate the cutter head. That’s when testers heard a noise and took Bertha apart. They found that some parts of the machine weren’t moving properly and caused “some metal-on-metal contact and basically damaged the seal assembly for the main bearing,” Preedy said."
This was a manufacturing problem discovered when it was first spun up in a test, as far as I can tell not even involving any digging, just making sure everything functioned correctly (e.g. what NASA didn't do with the Hubble mirror).
