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.
Livenet testing/shipping cycles could help explain some of it.
Test Lot 1 causes the first increase. Shutdown Lot 1 and ship causing the first decrease. During the second cycle, Test Lot 2 while at the same time Lot 1 is arriving at the purchasers and being powered on. Test Lot 2 gets shut down and shipped to end users. Test Lot 3 comes online for testing while Test Lot 2 arrives at the end-user.
Rinse. Repeat.
However, I think this scenario, while possible, is unlikely as a manufacturer wouldn't necessarily start/test/shutdown/ship in lots like this. They'd probably cycle HW in and out of deployment testing and shipping.