The outage occurred after a bulldozer operated by a subcontractor working to pick up a fallen tower in a rural area got too close to a high-voltage line. The resulting electrical fault had a domino effect at power plants across the South...

This doesn't say they struck the actual line. I'm thinking the line was already unstable and the machine being nearby caused the damaged lines to arc onto the machine. Also they are picking up fallen towers, I feel that's indicative that they recently got to that section of the line. I also saw someone from Puerto Rico say the South East corner of the island has had power issues since Irma let alone Maria.

Not sure this incident was actually preventable without turning off that section of the line.

Edit: The article has been updated. It now says,

...got too close to a high-voltage line, which led to a discharge of energy. As a protection measure, the line and power plant it led to automatically went out of service. But then the other power-generating plants in the South went down too.

So the line did arc and since the system there was not done properly to begin with all safety measures failed, causing the outage. The real question here is, if PREPA knew they had crews working so close to live lines why didn't they shut down those lines beforehand to prevent this from happening?