It was a glitch, as stated by BUNGIE 3 weeks ago. Was unintentional, they stated they would never do a ninja patch and all patches would be in the patch notes. They are going to patch it. When you have one more vex there at the bottom. Have one player go in the valley and they kill the last vex, then the next wave will spawn and you can go back up to snipe. Do it all three rounds. NOW THAT IT HAS BEEN EXPLAINED, STOP BITCHING AND CRYING