There are usually natural variations in difficulty. I've seen a drop like this before simply due to variance,
While its usual, I doubt its natural. I see a clear pattern that hashrate tends to drop in the second half of each difficulty window. Its almost as if someone is throttling his hashrate to prevent the next adjustment to be too large. That doesnt seem to make a lot of sense financially, so you could theorize the entity is diverting hashrate for a blockwitholding attack (is that still going on?)
