Alternative hypothesis: It seems to me that they're being drawn to these power cables like moths to a flame.

There's a comment above in which some guy observes that the strength of the EM field emitted by these cables is similar in strength to the Earth's EM field.

The article also mentions that these crabs migrate. Presumably they migrate using some organ that is sensitive to the earth's magnetic pull.

We know what happens when moths--who use light to navigate--encounter artificial lights. Their navigation hardware can't handle it, and they end up circling the light. This seems like the same kind of thing.