Possible motives:

A) Terrorists making a test run, checking the "cause and effects"?

Ask the FBI about this possibility.

B) One or more disgruntled employee(s) or former employee(s) getting a little payback?

Any recent layoffs from local or state telecommunications companies?

C) Someone(s) trying to draw attention to a weakness/security flaw in the system?

Eco-Terrorists, Extra right or left wing extremists, or an overzealous and overly concerned citizen's group?

D) The cable broke by itself due to: Earthquake 1's thru 3's/road traffic vibration/whatever else is possible?

What do you think?

A) Possible;
B) More likely than A);
C) Possibility between A) & B), not unheard of;
D) Per CDR Spock: "When all possibilities and probabilities have been exhausted, then the impossibile and improbable possibilities must be examined.