LOL, sadly the best written article on the subject of broken undersea cables comes from a NY Times article from 1903:
http://query.nytimes.com/mem/archive-free/pdf?_r=1&res=9E0DE1D91439E433A2575BC1A9669D946297D6CF
Apparently back in those days, if your anchor snagged an underseas cable, it was common practice to just ditch the anchor because the cable companies would buy you a new one rather than have the angry ship captain taking an axe to the cable to free his anchor.
That article makes more sense than my guess at the cause: The dreaded sawfish, nature's biological saw.
By marintime law you have to dump your anchor if it is snagged. You also risk sinking your ship if your anchor is snagged.