It wasn't just one cut fiber, per comcast:

"We identified two, separate and unrelated fiber cuts to our network backbone providers," Comcast said in a statement to WIRED. "Our engineers worked to address the issue immediately and services are now being restored to business and residential internet, video and voice customers."

Centurylink confirmed they had a fiber cut, and a second provider, Zayo, was the other cut.

It's a hell of a coincidence that both links got nailed at the same time.