Ofcourse the Predators were being hunted, the Xenomorph is as much a predator as the Yautja is. This only happened to look wrong because the pyramid was prematurely activated. There weren`t supposed to be aliens there only after the preds got their plasmacasters and were prepared for them, they were cought off-guard, that`s why it looked like they were the hunted. I do addmit that it was imbalanced, the way a single alien killed more than one predator in a single scene, because it is implied that the predators knew what they were doing going in there. It was supposed to be a ceremonial first kill, not a whole war, right? So it was supposed to be kind of an "easy difficulty" alien hunt. The situation got out of controll, otherwise no wrist bomb would have been detonated.