Even with slight, random muscle control over the anus, when said anus is viewed by an observer / sexual partner, the anus resembles the body of the spider. Moreover, the wrinkled folds of skin surrounding the anus ( or pucker) lend the appearance of an arachnid's limbs (i.e. a spider's legs) flailing about.
Naturally, said "arachnid" legs are thought to be moving in anger as the anus goes through various degrees of relaxation or tension, with the anger being a reaction to the threat of imminent exploitation that the anus is likely to experience at the hands of another participant.
Although the term is usually applied to women it may be used for men as well.