Biggest bitcoin mining pools all loosing hashrate

What happening? All of the big miners drop out about 48-24 hours before the mining difficulty increases.

If you look at the 24 hour change that if for overall hashrate for the particular pool. ant pool the largest pool which is owned and operated by bitmain. has lost an astonishing 17.86 percent and still has over a EH of hashing power. This is tactic the largest miners use to keep the difficulty down. Is this a good or a bad thing? Opinions?