Hey Guys,
Why did Bitcoin Cash difficulty increased about X 4 times in ~24 hours? Bug or something else?

It's "Error by design" which got abused by miners to mine by far more BCH blocks than they should (started parallel to the heavy pump of BCH via KRW/BCH pairing), now after some days the diff adjustment kicked in and the miners leave again as it's no longer profitable to mine BCH. As per current hash rate in 2 weeks and 2 days the diff will go down again and most likely the miners will
come back to abuse that "feature" to make a lot of money in short time

Just look e.g. at POW / Regarget, POW / Hashrate, Blocks / Blocks/hr, Reward / Opportunity costs, ... at
http://fork.lol Why abuse?
BCH difficulty was very low due to few post-fork EDAs, and many miners started switching after the price started increasing just before retarget.
The result was mining next 2016 blocks much quicker.
This, in turn, led to x4 diff increase for the next period.
So, this is not "Error by design" but rather "works as designed"