There are some various theories. There is a virus that is running though bees, which causes them to lose memory and sense of time and distance, so they cannot make it back to the hive. Bees are an excellent guage of the enviroment, and mirror humans quite well.
They are stressed, and no one is sure if the stress is causing the virus to be more effective, or the virus itself is the cause of increased stress. A new type of pesticide (which causes forgetfulness and laziness in insects), genetic engineered crops, pollinating only one type of crop in a litmited area so that they have to work harder for less pollen are all believed to be contributing factors. They basically have less and less free time, and are being forced to be ever bussier bees.