What I've read indicates the forager bees are dying away from the hive, the dead aren't being found in the hives vicinity. That tells me they got disoriented/lost. If foragers die off and then the nurse bees are forced into the forager role early (due to not being fed the pheromone) and then they die off for the same reason as the previous generation of foragers, who's left to do the nursing? The hive gets younger and younger until it cannot function.

There are lots of variables in this problem but it would seem that a controlled study should be able to varify or discount some of these hypothesis' quickly.