What could possibly explain this highly periodic crash in overall network hashing speed? I think this is not a random thing, but rather some effect due to actions taken by minors. What is going on? That is a pretty strong down trend late in the cycle.
It's just statistical noise. No-one would rationally turn off mining capacity and then switch it back on and the power requirements to make a 10% dent in the hash rate are huge (> 10 MW and probably closer to 20 MW).
Here's the last year in log form (which shows the relative size of swings evenly across the last year):
https://blockchain.info/charts/hash-rate?timespan=1year&showDataPoints=false&daysAverageString=1&show_header=true&scale=1&address=
I can't see anything that's dramatically different about the last 4 difficulty periods than the preceding 30-ish and all are well within predictable statistical norms.