Personally speaking,,,as someone else has already suggested, this sounds like a test run to test the feasibility and effectiveness of such an act of sabotage.
Maybe I'm not paranoid enough but it seems like if someone were testing the system doing it like this would just tip their hand to the authorities, making it harder to do the same thing when it came time for the real strike.
I wonder if it was actually a warning. You know, somebody trying to point out just how easy it would be to bring it all down. It seems they could have made it a lot worse beyond just cutting the cables. Cutting them followed by torching the place would have made it a lot more difficult to repair.
-Blast