This person might have huge computing power and could mine a huge number of blocks when the difficulty is low, driving the difficulty way up while extracting easy blocks, and then stop mining and let others mine at a much higher difficulty, then mine again when the difficulty is lowered and continue the cycle
That's probably from the fork - if the network was cut in half, then half the mining output during the fork got thrown away (in practice, it won't be that much, because there's no reason for exactly half to be on each part). So the diff drop will overcompensate this time, and then it'll bounce pretty high next time, and then start settling back down to normal for the amount of mining power that's out there.
But didn't the fork and its consequences already stop? I don't think that many miners are still mining for a wrong blockchain
The difficulty increased to an all time high of 1804 (except super blocks) and now the average block find time is about 330 sec (more than 5 minutes)!
And the hashrate distribution of "All others" here for the last 100 blocks is only 8 %, but is 71.7 % for the last 1000 blocks! I don't think that this drop is a consequence of the fork
I would think that a miner with huge computing resources stopped mining right after the difficulty rise, and will not be surprised if this person/organization resumes mining when the difficulty decreases again
I think we're both right.
The fork caused the diff drop, by making it seem as if the hashrate was actually only about 60% of what it was.
The diff dropped too low for the real hashrate, so after the fork healed, the hashrate was huge for the diff. That caused the diff spike.
And with the diff spike, some huge miner(s?) dropped out.
And you're right - this is the kind of diff swing that newer diff salgorithms like DGW were designed to prevent. Let's hope it settles down for now.