I suspect the rippling and waving of the poncho resembles something unseen moving through tall grass. And in the poncho case masking the identity or even humanness of its wearer.

Evolutionary pressure from http://redwinggreen7.livejournal.com/96599.html#cutid2-end, what else?

It seems to have worked! Bovine are plentiful, but you rarely see cloakers :-D