There is some discussion to be had as to whether there are other sections of water that are more active and have been for some years, but even if the channel isn't the busiest it is certainly very busy.
-Educated guess is that cargo vessels aren't much of a problem. Anything dropping anchor, dredging or somehow doing stuff on the bottom, on the other hand...
Further, I'd wager that initially buried cable in the Channel often is exposed on the bottom due to strong currents, rendering it more vulnerable - but this is just a guess, mind.
How often? Once or twice a year?
And what were the reason it breaking so often? I thought the English Channel were quite free of Cargo Ships and the like.