Could also be shoddy workmanship. Country of origin for the workman is at best a vague hint of quality, and a poor construction can take years if not decades to fail. Keystone's XL decommissioning planned penned out that they expected a minimum of 50 years of service from that pipeline, but over half the pipelines in the US carrying oil and gas are in excess of 60 years old and still operating.

Until the investigation is perform though, we simply don't have enough to go on.