According to
http://bitcoin.sipa.be/, it seems some big miners will purposely slow down the hashing just before the difficulty adjusting to reduce the jump? Although I know that does not make much sense, since what they get is based on their hash rate percentage, not the difficulty. Maybe they are just doing that to save the environment.
I'm seeing the opposite on that graph. Looks like the difficulty growth levels out for a day or so after each adjustment. That would be consistent with people looking at their mining revenue after an adjustment and deciding to shut down their rigs.
I think both of you are seeing patterns where none exist (which is a typical human thing to do).
There is no incentive to shut down hashing just prior to a difficulty adjustment. The effect on the new difficulty is minimal and you lose more from lost mining time than from the marginally lower difficulty.
Additionally, the only reason to shut down a rig is when the electricity cost outweighs the income. While many rigs may not be profitable anymore in the sense that they won't earn back their purchase price, all ASICs are still making more coins than they cost to keep running, so there's no reason to turn off an ASIC that you have running. And by now, the fraction of GPU-miners is so low that GPUs being turned off isn't going to make a noticeable dent in the charts.