[spkthed]

Materials act differently under different conditions. Lots of pressure can make some materials very brittle or even softer. Brittle materials plus the weight/stress from the water currents involved could be what they are referring to.

I don't know the specifics of how the steel/iron/other compounds would react to that pressure but that's a guess.

Edit:

Other sources leave out the pressure part, it looks like the weight of the cable would be the issue. I guess it's also why we don't have a space elevator yet.