From my own observations, the recent drop is sure to uncertainty with Python 3, and an increase of alternatives on server side, such as Node.

The transition is only going to get more painful as system critical software lags on 2.x while users clamour for 3.x. I understand there are some fundamental problems in running both simultaneously which makes gradual integration not a possibility. Dynamic typing also doesn't help, making it very hard to automatically port, and update dependencies.

Lesser reasons include an increasing gap in scalability to multicore compared with other languages (the GIL being the gorilla here, multiprocessing is unacceptable as long as native threading is the only supported concurrency mechanism), and a lack of enthusiasm from key technologies and vendors: GAE, gevent, matplotlib, are a few encountered personally.