Heedm,
The letter that I referred to was from TRW, written by their engineer following an investigation of a damaged winch cable operated by an Australasian company who used the hook as an anchor point for rapelling practice. Friends in Queensland with a better filing system than mine provided me with another copy of the letter, but I am waiting on the author's permission before posting it verbatim.
However, since it was distributed publicly to most Australian operators, the gist of the letter was that the shock load on the hook has an adverse effect on the short length of cable to the drum, which is unable to absorb the strain, thus "unwrapping" the helix of the cable. The main strain acts on the inner core of the cable, where it can't be seen, and further shock loads can then result in a broken cable. If you suspect that your cable has been used for rapelling/fast roping, a check with a micrometer to ascertain cable thickness is strongly recommended.
If you want the letter, let me know and I can e mail it.