Their upcoming IPO makes a lot more sense now.
They have been propping up bitcoin cash price by (1) requiring ASIC purchases to be made w/ bch (2) not selling the bch they earn (3) actively buying bch with btc and (4) protecting the chain from 51% attacks.
As a result, Bitmain currently has a ~$500M bag of ~1M bitcoin cash that they can't sell without cratering the price. Even just selling a fraction could spook the bch market.
The IPO is a way of 'selling it' without selling it.