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I have no idea why the sudden divergence in profitability. Anyone?
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Antpool mining empty blocks again

With these often swings between chains is slowly lowering the BTC difficulty which means the BCH chain needs to run at a lower and lower difficulty just to remain profitable.
It will get to the point where the BCH difficulty is so low to remain profitable especially if the BTC value keeps increasing that they will be a easy target for any miner that wants to do malicious things

There basically backing themselves into a corner just hoping no one gives them a beating