By last November, Hirotaka Yokota had finally had enough.

He told Yamato Transport that his drivers would no longer ferry packages. The nation's largest parcel carrier was working them into the ground, with a surging volume of packages from online retailers such as to homes and businesses. And they could get better-paying work elsewhere, without doing 15-hour shifts.

"More and more subcontractors were having that conversation," said Yokota, who runs Artpla Ltd., a former delivery subcontractor for Yamato. "Yamato ran out of drivers and was relying on its own employees. And then they couldn't manage."