From what little I have read on the subject, it may have something to do with parental investment and mode of fertilization. In cases of internal fertilization, males are guaranteed that their offspring will receive a certain amount of care, and the pressure is on them to take off. In cases of external fertilization, there is opportunity for males to start doing the productive work, and roles can be reversed more easily.
In "Animal Behavior" by
Mark Ridley, Ridley says that he informally surveyed all of the organisms that have polyandrous mating systems, and that there was a clear correlation between mode of fertilization and mating system. This survey - besides not being a formal study- is also way out of date by now(my copy is 14 years old). Google scholar turns up some more hits for "polyandry external fertilization" and such, so that might be a good place to start if you have journal access.
Also, I happened to stumble upon this PBS page on Jacana polyandry on google, might be of interest:
Evolution: Library: Jacanas and Polyandry
