The current oscillation in hash power looks like a death spiral. What you are seeing is one or more miners/pools only mining when difficulty is low, and then turning off or mining another chain when difficulty increases in response to their mining. This leads to significantly increased block times when the difficulty is high and the miners are elsewhere and useless ridiculously fast block times when difficulty is low and they return. The periods of higher difficulty form a reinforcing disincentive for other miners to continue mining during those periods. This is what we call a feedback loop.