Robert Krulwich explores why it happens, why evolution hasn't stopped it, and why it makes him nervous:
These ants are completely blind so they get about by sniffing trails left by the ants in front of them. They, in turn, leave chemical trails of their own. The system works smoothly when everybody's going in a straight line in one direction... But when the lead ants start to loop, bad things can happen (and remember we humans loop too, we can't hold a straight course over long distances without external points of reference). If the ant-in-front loops and intersects with its old trail, the whole crowd then turns in on itself and everybody gets caught in the endless circle.